Photonic crystal fibers having a preferred bending plane and systems that use such fibers

ABSTRACT

In general, in a first aspect the invention features photonic crystal fibers that include a core extending along a waveguide axis, a confinement region extending along the waveguide axis surrounding the core, and a cladding extending along the waveguide axis surrounding the confinement region, wherein the cladding has an asymmetric cross-section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of and claimspriority under 35 U.S.C. §120 to U.S. application Ser. No. 11/101,915.entitled “PHOTONIC CRYSTAL FIBERS AND MEDICAL SYSTEMS INCLUDING PHOTONICCRYSTAL FIBERS,” filed on Apr. 8, 2005. This application also claimspriority under 35 U.S.C. §119(e)(1) to Provisional Patent ApplicationNo. 60/658,531, entitled “PHOTONIC CRYSTAL FIBERS,” filed on Mar. 4,2005. The entire contents of both, of the above-mentioned applicationsare incorporated herein by reference.

BACKGROUND

This invention relates to the field of photonic crystal waveguides andsystems using photonic crystal waveguides.

Waveguides play important roles in numerous industries. For example,optical waveguides are widely used in telecommunications networks, wherefiber waveguides such as optical fibers are used to carry informationbetween different locations as optical signals. Such waveguidessubstantially confine the optical signals to propagation along apreferred path or paths. Other applications of optical waveguidesinclude imaging applications, such as in an endoscope, and in opticaldetection. Optical waveguides can also be used to guide laser radiation(e.g., high intensity laser radiation) from a source to a target inmedical (e.g., eye surgery) and manufacturing (e.g., laser machining andforming) applications.

The most prevalent type of fiber waveguide is an optical fiber, whichutilizes index guiding to confine an optical signal to a preferred path.Such fibers include a core region extending along a waveguide axis and acladding region surrounding the core about the waveguide axis and havinga refractive index less than that of the core region. Because of theindex-contrast, optical, rays propagating substantially along thewaveguide axis in the higher-index core can undergo total internalreflection (TIR) from the core-cladding interface. As a result, theoptical fiber guides one or more modes of electromagnetic (EM) radiationto propagate in the core along the waveguide axis. The number of suchguided modes increases with core diameter. Notably, the index-guidingmechanism precludes the preserve of any cladding modes lying below thelowest-frequency guided mode for a given wavevector parallel to thewaveguide axis. Almost all index-guided optical fibers in usecommercially are silica-based in which one or both of the core andcladding are doped with impurities to produce the index contrast andgenerate the core-cladding interface. For example, commonly used silicaoptical fibers have indices of about 1.45 and index contrasts rangingfrom about 0.2% to 3% for wavelengths in the range of 1.5 mm, dependingon the application.

Another type of waveguide fiber, one that is not based on TIRindex-guiding, is a Bragg fiber, which includes multiple alternatingdielectric layers surrounding a core about a waveguide axis. Themultiple layers form a cylindrical mirror that confines light to thecore over a range of frequencies. The alternating layers are analogousto the alternating layers of a planar dielectric stack reflector (whichis also known as a Bragg mirror). The multiple layers form what is knownas a photonic crystal, and the Bragg fiber is an example of a photoniccrystal fiber, Photonic crystal structures, are described generally inPhotonic Crystals by John D, Joannopoulos et al. (Princeton UniversityPress, Princeton N.J., 1995).

Drawing a fiber from a preform is the most commonly used method formaking fiber waveguides. A preform is a short rod (e.g., 10 to 20 incheslong) having the precise form and composition of the desired fiber. Thediameter of the preform, however, is much larger than the fiber diameter(e.g., 100's to 1000's of times larger). Typically when drawing anoptical fiber, the material composition of a preform includes a singleglass having varying levels of one or more dopants provided in thepreform core to increase the core's refractive index relative to thecladding refractive index. This ensures that the material forming thecore and cladding are theologically and chemically similar to he drawn,while still providing sufficient index contrast to support guided modesin the core. To form the fiber from the preform a furnace heats thepreform to a temperature at which the glass viscosity is sufficientlylow (e.g., less than 10⁸ Poise) to draw fiber from the preform. Upondrawing, the preform necks down to a fiber that has the samecross-sectional composition and structure as the preform. The diameterof the fiber is determined by the specific rehological properties of thefiber and the rate at which it is drawn.

Preforms can be made using many techniques known to those skilled in theart, including modified chemical vapor deposition (MOVD), outside vapordeposition (OVD), plasma activated chemical vapor deposition (PCVD) andvapor axial deposition (VAD). Each process typically involves depositinglayers of vaporized raw materials onto a wall of a pre-made tube or rodin the form of soot. Each soot layer is fused shortly after deposition.This results in a preform tube that is subsequently collapsed into asolid rod, over jacketed, and then drawn into fiber.

Optical fibers applications can be limited by wavelength and signalpower. Preferably, fibers should be formed from materials that have lowabsorption of energy at guided wavelengths and should have minimaldefects. Where absorption is high, it can reduce signal strength tolevels indistinguishable from noise for transmission over long fibers.Even for relatively low absorption materials, absorption by the coreand/or cladding beats the fiber. Detects ears scatter guided radiationout of the core, which can also lead to heating of the fiber. Above acertain power density, this heating can irreparably damage the fiber.Accordingly, many applications that utilize high power radiation sourcesuse apparatus other than optical fibers to guide the radiation from thesource to its destination.

SUMMARY

In general, in a first aspect, the invention features a photonic crystalfiber that includes a core extending along a waveguide axis, aconfinement region extending along the waveguide axis, the confinementregion surrounding the core, and a cladding extending along thewaveguide axis, where the cladding surrounds the confinement region. Thecladding has an asymmetric cross-section that extends along a length ofthe photonic crystal fiber.

Embodiments of the-photonic crystal fiber can include one or more of thefollowing features. For example, the confinement region can include alayer of a first material arranged in a spiral structure that extendsalong the waveguide axis and the asymmetric cross-section causes thephotonic crystal fiber to bend preferably in a plane that does notintersect an end of the spiral structure that is adjacent the core. Thephotonic crystal fiber can be configured to guide radiation at awavelength λ along the waveguide axis where the confinement regionincludes a periodic structure that substantially confines the radiationto the core. The cladding can include a layer of a first materialsurrounding the confinement region, the layer having a thickness along adirection normal to the waveguide axis that is larger than the period ofthe periodic structure of the confinement region (e.g. the layerthickness can he about 10 times larger than the period, about 20 timeslarger, about 50 times larger, about 100 times larger, about 200 timeslarger, about 400 times larger).

In certain, embodiments, the asymmetric cross-section causes thephotonic crystal fiber to bend preferably in a bend plane relative toother planes.

The confinement region can include a seam extending along the waveguideaxis. In some embodiments, the confinement region includes a layer of afirst material that is arranged in a spiral around the waveguide axisand the seam is the end of the layer that is adjacent the core. Thecladding can have a short cross-sectional dimension, a, non-coincidentwith the seam. The seam can be located in a range from about 80 degreesto about 110 degrees from, the short cross-sectional dimension. Thecladding can have a short cross-sectional dimension, a, and a longcross-sectional dimension, b, and an ellipticity, ε, given by theformula:

${ɛ = \frac{\left( {b - a} \right)}{\frac{1}{2}\left( {b + a} \right)}},$

that is in a range from about 0.05 to about 0.5 (e.g., about 0.08 ormore, about 0.1 or more, about 0.12 or more, about 0.15 or more, about0.2 or more, about 0.4 or less, about 0.3 or less, such as about 0.25).

The confinement region can include a layer of a first dielectricmaterial arranged in a spiral around the waveguide axis. The confinementregion can further include a layer of a second dielectric materialarranged in a spiral around the waveguide axis, the second dielectricmaterial having a different refractive index from the first dielectricmaterial. The first dielectric material can be an inorganic dielectricmaterial, such as a glass (e.g., a chalcogenide glass). The seconddielectric material can be an inorganic dielectric material, such as apolymer.

In some embodiments, the confinement region includes at least one layerof a chalcogenide glass. In certain embodiments, the dielectricconfinement region includes at least one layer of a polymeric material.The core can be a hollow core.

In some embodiments, the photonic crystal fiber is configured to guideradiation at about 10. 6 μm along the waveguide axis.

In another aspect, the invention features a system that includes a CO₂laser and the foregoing photonic crystal fiber. The photonic crystalfiber has an input end that is positioned relative to the CO₂ laser toreceive radiation from the CO₂ laser and the photonic crystal fiberbeing arranged to deliver the radiation to a target.

In a further aspect, the invention features a system that includes theforegoing photonic crystal fiber which has an input end and an outputend, and a handpiece attached to the photonic crystal fiber. Thehandpiece allows an operator to control the orientation of the outputend to direct the radiation to a target location of a patient. In someembodiments, the handpiece includes an endoscope. The endoscope caninclude a flexible conduit and a portion of the photonic crystal fiberis threaded through a channel in the flexible conduit. The endoscope caninclude an actuator mechanically coupled to the flexible conduitconfigured to bend a portion of the flexible conduit in at least oneplane thereby allowing the operator to vary the orientation of theoutput end. The photonic crystal fiber can be attached to the endoscopeso that the at least one plane corresponds to the bend plane of thephotonic crystal fiber.

In general, in another aspect, the invention features a photonic crystalfiber that includes a core extending along a waveguide axis, aconfinement region extending along the waveguide axis, the confinementregion surrounding the core, and a cladding extending along thewaveguide axis, the cladding surrounding the confinement region. Thephotonic crystal fiber bends preferably in a bend plane relative toother planes.

Embodiments of the photonic crystal fiber can include one or more of thefollowing features and/or one or more features of other aspects.

For example, in some embodiments, the cladding includes a first portionextending along the waveguide axis and a second portion extending alongthe waveguide axis, the first portion being composed of a first materialand the second portion being composed of a second material having agreater stiffness than the first material. The cladding can furtherinclude a third portion being composed of a third material having agreater stiffness than the first material. In cross-section, the corecan be positioned between the second portion and the third portion. Thefirst portion can surround the second portion.

In general, in a further aspect, the invention features a fiberwaveguide that includes a core extending along a waveguide axis, a firstportion extending along the waveguide axis, the first portion,surrounding the core, and a cladding extending along the waveguide axis,the cladding surrounding the first portion region. An interface betweenthe core and the first portion includes a defect (e.g., a seam) thatextends along the waveguide axis and the fiber waveguide bendspreferably in a bend plane relative to other planes. Embodiments of thephotonic crystal fiber can include one or more features of otheraspects.

In general, in a further aspect, the invention features a fiberwaveguide that includes a core extending along a waveguide axis, a firstportion extending along the waveguide axis, the first portionsurrounding the core, and a cladding extending along the waveguide axis,the cladding surrounding the first portion, wherein an interface betweenthe core and the first portion includes a defect (e.g., a seam) thatextends along the waveguide axis and the cladding has an asymmetriccross-section that extends along a length of the photonic crystal fiber.Embodiments of the photonic crystal fiber can include one or morefeatures of other aspects.

In general, in a further aspect, the invention features a fiberwaveguide that includes a core extending along a waveguide axis, and afirst portion extending along the waveguide axis, the first portionsurrounding the core, wherein the first portion has an asymmetriccross-section that extends along a length of the photonic crystal fiber.Embodiments of the photonic crystal fiber can include one or morefeatures of other aspects.

In general in another aspect, the invention features photonic crystalfibers that include a core extending along a waveguide axis, aconfinement region extending along the waveguide axis surrounding thecore, and a cladding extending along the waveguide axis surrounding theconfinement region, wherein the cladding has an asymmetriccross-section.

In general, in another aspect, the invention features photonic crystalfibers that include a core extending along a waveguide axis, aconfinement region extending along the waveguide axis surrounding thecore, and a cladding extending along the waveguide axis surrounding theconfinement region, wherein the photonic crystal fiber bends preferablyin a first plane compared to other planes.

In general, in a further aspect, the invention features photonic crystalfibers that include a core extending along a waveguide axis, aconfinement region extending along the waveguide axis surrounding thecore, and a cladding extending along the waveguide axis surrounding theconfinement region, the cladding having a first diameter of a first sizeand a second diameter of a second size different from the first size.

In general, in another aspect, the invention features photonic crystalfibers that include a core extending along a waveguide axis, aconfinement region extending along the waveguide axis surrounding thecore, and a cladding extending along the waveguide axis surrounding theconfinement region, the cladding having a surface with a cross-sectionhaving portions with differing radii of curvature.

Embodiments of the photonic crystal fibers can include one or more ofthe following features. The confinement region can include a seam. Theseam can he non-coincident with the first plane. The seam can beadjacent the core. The cladding can have a short cross-sectionaldimension non-coincident with the seam. The seam can be located about 80degrees or more from the cross-sectional dimension. The seam can belocated about 85 degrees or more from the cross-sectional dimension. Theseam can be located about 90 degrees from the cross-sectional dimension.The confinement region can have a spiral cross-section. The confinementregion can include a chalcogenide glass.

Among other advantages, the photonic crystal fibers can control whichportion of a fiber is on the inside or outside of a bend in the fiber.For example, fibers can be designed so that the fiber preferably bendsin a way that a seam (or other defect) in the fiber is not positioned onthe outside of the bend. Positioning a defect away from the outside of abend cm reduce loss of guided radiation due to the bend, and can reducefiber failure due to, e.g., heating of the fiber at the defect when thedefect is positioned on the outside of a bend.

Accordingly, fibers can be provided that have improved losscharacteristics compared with fibers that don't have a preferred bendplane. Improved loss characteristics can result in higher workingpowers, greater efficiency, and/or longer working lifetimes. Improvedloss characteristics can also allow fibers to he used in applicationsnot previously appropriate for the fibers, such as certain high powerapplications (e.g., high power medical applications).

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features andadvantages of the invention will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view of an embodiment of a photonic crystalfiber.

FIG. 1B is a perspective view of the embodiment of the photonic crystalfiber shown in FIG. 1A.

FIGS. 2A-2E are schematic diagrams showing stages in a method forforming the photonic crystal fiber shown in FIG. 1.

FIG. 3 is a cross-sectional view of an embodiment of a photonic crystalfiber.

FIGS. 4A-4D are cross-sectional views of embodiments of photonic crystalfibers.

FIG. 5 is a schematic diagram of a medical laser system that includes aphotonic crystal fiber.

FIG. 6A is a schematic diagram of a medical laser system that includes aphotonic crystal fiber and an endoscope.

FIG. 6B is a schematic diagram of the endoscope shown in FIG. 6A.

FIG. 7 is a schematic diagram of an optical telecommunication systemthat implements photonic crystal fibers described herein.

FIG. 8 is a schematic diagram of a laser system that implements photoniccrystal fibers described herein.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1A and 1B, a photonic crystal fiber 100 includes acore 120 extending along an axis 199, and a confinement region 110surrounding the core. A cladding 160 surrounds the confinement region.Photonic crystal fiber 100 is configured to guide radiation at a guidingwavelength λ along axis 199. As discussed below, confinement region 110substantially confines guided radiation at 1 to core 120. Cladding 160serves primarily to protect and mechanically support, confinement region110.

Cladding 160 has an asymmetric cross-section with a larger diameteralong a major axis 161 compared to its diameter along a minor axis 162orthogonal to the major axis. The major and minor axes are orthogonal toaxis 199. The asymmetric cross-section is also manifested in the shapeof the cladding's outer surface. In particular, the outer surface ofcladding 160 includes portions of differing curvature. In particular,cladding 160 includes arcuate portions 131 and 132 and two straightportions 133 and 134. Arcuate portions 131 and 132 are on opposite sidesof the cladding along major axis 121. Straight portions 133 and 134 areon opposite sides of the cladding along minor axis 122. Cladding 160 isco-drawn with confinement region 110 when the fiber is produced.

In general, the asymmetry of the cross-sectional profile of cladding 160is sufficient to cause fiber 100 to preferably bend in a plane 101defined by fiber axis 199 and the minor axis 162 during normal use ofthe fiber. In general, where a fiber has a plane in which the resistanceto bending is less than other planes, the plane is referred to as a“bend plane.”

The ratio of cladding 160's outer diameter, b₁₆₀, along the major axisto its outer diameter, a₁₆₀, along the minor axis can vary. Typically,this ratio is selected so that fiber 100 bends preferably in the bendplane, while cladding 100 still provides the desired mechanical supportor other function(s) for which it is designed (e.g., opticalfunction-thermal management), in some embodiments, this ratio can herelatively low, soon as about 1.5:1 or less (e.g., about 1.4:1 or less,about 1.3:1 or less, about 1.2:1 or less, about 1.1:1 or less).Alternatively, in certain embodiments, this ratio can be larger thanabout 1.5:1. (e.g., about 1.6:1 or more, about 1.7:1 or more, about1.8:1 or more, about 1.9:1 or more, about 2:1 or more).

The ratio of b₁₆₀ to a₁₆₀ can be characterized as an ellipticity; ε,which is mathematically expressed as:

$ɛ = {\frac{\left( {b_{160} - a_{160}} \right)}{\frac{1}{2}\left( {b_{160} + a_{160}} \right)}.}$

Typically, ε is selected so that fiber 100 has desired mechanicalproperties, ε is generally sufficient large so that fiber 100 has apreferred bend plane. For example, ε can be about 0.05 or more (e.g.,about 0.08 or more, about 0.10 or more, about 0.12, or more, about 0.15or more, about 0.1.8 or more, about 0.20 or more, about 0.22 or more), εshould not he so large that if introduces unwanted ellipticity intoother parts of the fiber, such as the core. In some embodiments, ε isless than 0.50 (e.g., about 0.40 or less, about 0.30 or less, about 0.25or less, about 0.20 or less). In certain embodiments, ε is in a rangethat provides a preferred bend plane, but does not entirely prevent thefiber bending in the plane orthogonal to the preferred bend plane. ε canbe in a range from about 0.08 to about 0.25 (e.g., from about 0.1.0 toabout 0.20, from about 0.12 to about 0.18).

Typically, sis substantially constant along the length of fiber 100,However, in certain embodiments, e can vary along the length of thefiber. For example, in some embodiments. It may be desirable to have abend plane in one section of a fiber but not in another. In such cases,ε can he relatively large in the section where a bend plane is desired,but small or zero in other sections. Further, in certain embodiments, aand b can be oriented differently with respect to a referenceco-ordinate system for different portions of a fiber. For example, insome embodiments, it may be desirable to have a bend plane in oneorientation in one section of a fiber, while having a bend plane with adifferent orientation at another section. This can be achieved by havinga and b oriented differently in the different sections of the fiber.Vary ε and/or a and b orientation can be achieved introducing theasymmetry into the preform, while drawing fiber from the preform, orafter the fiber has been drawn.

In some embodiments, two or more lengths of fibers having differing ε'sand/or differing orientations of a and b may be connected to provide aconcatenated fiber that has different mechanical properties along itslength.

In general, the actual dimensions of a₁₆₀ and b₁₆₀ can vary depending onthe operational wavelength of operation of fiber 100 and otherconstraints imposed by the application for which the fiber is used. Forexample, a₁₆₀ and b₁₆₀ should be sufficiently larger to provide adequatemechanical support and protection for core 120 and confinement region110, However, a₁₆₀ and b₁₆₀ should be small enough so that the fiber issufficiently flexible and/or capable in fitting in fiber conduits of aparticular size (e.g., in an endoscope conduit). In some embodiments,a₁₆₀ and/or b₁₆₀ are about 500 μm or more (e.g., about 750 μm or more,about 1,000 μm or more, about 1,250 μm or more, about 1,500 μm or more,about 1,750 μm or more, about 2,000 μm or more). a₁₆₀ and/or b₁₆₀ can beabout 10,000 μm or less (e.g., about 7,000 μm or less, about 5,000 μm orless, about 3,000 μm or less, about 2,000 μm or less).

Confinement region 110 includes continuous layers 130, 140, and 150 ofdielectric material (e.g., polymer, glass) having different refractiveindices, as opposed to multiple discrete, concentric layers that formconfinement regions in other embodiments. Continuous layers 130, 140,and 150 form a spiral around an axis 199 along which the photoniccrystal fiber waveguide guides electromagnetic radiation. One or more ofthe layers, e.g., layer 140 and/or layer 150, is a high-index layerhaving an index n_(H) and a thickness d_(H), and the layer, e.g., layer130, is a low-index layer having an index n_(L) and a thickness d_(L),where n_(H)>n_(L) (e.g., n_(H)−n_(L) can be greater than or equal to orgreater than 0.01, 0.05, 0.1, 0.2, 0.5 or more).

Because layers 130, 140, and 150 spiral around axis 109, a radialsection extending from axis 199 intersects each of the layers more thanonce, providing a radial profile that includes alternating high indexand low index layers. In some embodiments, layers 140 and 150 have thesame refractive index. In such cases, for all hut the innermost spiralof layer 140 and the outermost spiral of layer 150, adjacent layers 140and 150 effectively create a single layer (e.g., a single high index orlow index layer) along a radial section.

Confinement region 110 has an inner seam 121 and an outer seam 122corresponding to the edges of the continuous layers from which theconfinement region is formed. Inner seam 121 is located along an azimuth123 that is displaced by an angle a from minor axis 162. α can be in arange from about 10° to about 170° (e.g., from about 20° to about 160°,from about 30° to about 150° from about 40° to about 140°, from about50° to about 130°, from about 60° to about 120°, from about 70° to about110°, from about 80° to about 100°). in some embodiments, α is about90°.

The inner seam does not lie in bend plane 101 of the fiber. In fiber100, this, is achieved by locating inner seam 121 away from the minoraxis. Locating the inner seam away from the bend plane can beadvantageous since it is believed that losses (e.g., due to scattering,and/or absorption) of guided radiation is higher at the seam compared toother portions of the confinement region. Further, it is believed thatthe energy density of guided radiation in the core is higher towards theoutside of a bend in the fiber relative to the energy density at otherparts of the core. By locating the inner seam relative to the minor axisso that the seam is unlikely to lie in the bend plane (e.g., where α isabout 90°), the probability that the inner seam will lie towards theoutside of a fiber bend is reduced. Accordingly, the compounding effectof having a relatively high loss portion of the confinement region atthe region, where the energy density of guided radiation is high can beavoided, reducing the loss associated with bends in the fiber.

Although inner seam 121 and outer seam 122 are positioned at the sameazimuthal position with respect to axis 199 in fiber 100, its otherembodiments the inner and outer seams can be located along at differentrelative azimuthal positions with respect to the fiber's axis.

The spiraled layers in confinement region 110 provide a periodicvariation in the index of refraction along a radial section, with aperiod corresponding to the optical thickness of layers 130, 140, and150. Is general, the radial periodic variation has an optical periodcorresponding to n₁₃₀d₁₃₀+n₁₄₀d₁₄₀+n₁₅₀d₁₅₀.

In embodiments where layers 140 and 150 have the same refractive index,n_(H), and a combined thickness d_(H), layer 130 has a refractive indexn_(L) and thickness d_(L), confinement region 110 has an optical periodn_(H)d_(H)+n_(L)d_(L). The thickness (d_(H) and d_(L)) and opticalthickness (n_(H)d_(H) and n_(L)d_(L)) of layers 140 and 150 and of layer140 can vary. In some embodiments, the optical n_(H)d_(H)=n_(L)d_(L).Layer thickness is usually selected based on the desired opticalperformance of the fiber (e.g., according to the wavelength radiation tobe guided). The relationship between layer thickness and opticalperformance is discussed below. Typically, layer thickness is in thesub-micron to tens of micron range. For example, d_(L) and/or d_(H) canbe between about 0.1 μm to 20 μm thick (e.g., about 0.5 to 5 μm thick).

For the embodiment shown in FIG. 1, confinement region 110 is 5 opticalperiods thick. In practice, however, confinement region 110 may includemany more optical periods (e.g., more than about 8 optical periods, 10optical periods, 15 optical periods, 20 optical periods, 25 opticalperiods, such as 40 or more optical periods).

Layer 140 and 150 include a material that has a high refractive index,such as a chalcogenide glass. Layer 130 includes a material having arefractive index lower than the high index material of layers 140 and150, and is typically mechanically flexible. For example, layer 130often includes a polymer. Preferably, the materials forming layers 130,140, and 150 can be co-drawn. Criteria for selecting materials that canbe co-drawn are discussed below.

In the present embodiment, core 120 is hollow. Optionally, the hollowcore can be filled with a fluid, such as a gas (e.g., air, nitrogen,and/or a noble gas) or liquid (e.g., an isotropic liquid or a liquidcrystal). Alternatively, core 120 can include any material orcombination of materials that are ideologically compatible with thematerials forming confinement region 110. In certain embodiments, core120 can include one or more dopant materials, such as those described inU.S. patent application Ser. No. 10/121,452, entitled “HIGHINDEX-CONTRAST FIBER WAVEGUIDES AMD APPLICATIONS,” filed Apr. 12, 2002and now published under Pub. No. US-2003-0044158-A1, the entire contentsof which, are hereby incorporated by reference.

Core and confinement regions 120 and 110 may include multiple dielectricmaterials having different refractive indices. In such cases, we mayrefer to an “average refractive index” of a given region, which refersto the sum of the weighted indices for the constituents of the region,where each index is weighted by the fractional area in the region of itsconstituent. The boundary between layers 130 and 140 and layers 130 and150, however, are defined by a change in index. The change may be causedby the interface of two different dielectric materials or by differentdopant concentrations in the same dielectric material (e.g., differentdopant concentrations in silica).

Dielectric confinement region 110 guides EM radiation in a first rangeof wavelengths to propagate in dielectric core 120 along waveguide axis199. The confinement mechanism is based on a photonic crystal structurein region 110 that forms a bandgap including the first range ofwavelengths. Because the confinement mechanism is hot index-guiding, itis not necessary for the core to have a higher index than that of theportion of the confinement region immediately adjacent the core. To thecontrary, core 120 may have a lower average index than that ofconfinement region 110. For example, core 120 may be air, some othergas, such as nitrogen, or substantially evacuated. In such a case, EMradiation guided in the core will have much smaller losses and muchsmaller nonlinear interactions than EM radiation guided in a silicacore, reflecting the smaller absorption and nonlinear interactionconstants of many gases relative to silica or other such solid material.In additional embodiments, for example, core 120 may include a porousdielectric material to provide some structural support for thesurrounding confinement region while still defining a core that islargely air. Accordingly, core 120 need not have a uniform index,profile.

Layers 130, 140 and 150 of confinement region 110 form what, is known asa Bragg fiber. The periodic optical structure of the spirally woundlayers are analogous to the alternating layers of a planar dielectricstack reflector (which is also known as a Bragg mirror). The layers ofconfinement region 110 and the alternating planar layers of a dielectricstack reflector are both examples of a photonic crystal structure.Photonic crystal structures are described generally in Photonic Crystalsby John D. Joannopoulos et al. (Princeton University Press, PrincetonN.J. 1995).

As used herein, a photonic crystal is a dielectric structure with arefractive index modulation that produces a photonic bandgap in thephotonic crystal. A photonic bandgap, as used herein, is a range ofwavelengths (or inversely, frequencies) in Which there are no accessibleextended (i.e., propagating, non-localized) states in the dielectricstructure. Typically the structure is a periodic dielectric structure,but it may also include, e.g., more complex “quasi-crystals.” Thebandgap can be used to confine, guide, and/or focalize light bycombining the photonic crystal with “defect” regions that deviate fromthe bandgap structure. Moreover, there are accessible extended statesfor wavelengths both below and above the gap, allowing light to beconfined even in lower-index regions (in contrast to index-guided thestructures, such as those described above). The term “accessible” statesmeans those states with which coupling is not already forbidden by somesymmetry or conservation law of the system. For example, intwo-dimensional systems, polarization is conserved, so only states of asimilar polarization need to be excluded from the bandgap. In awaveguide with uniform, cross-section (such as a typical fiber), thewavevector β is conserved, so only states with a given β need to beexcluded from the bandgap to support photonic crystal guided modes.Moreover, in a waveguide with cylindrical symmetry, the “angularmomentum” index m is conserved, so only modes with the same m need to beexcluded front the bandgap. In short, for high-symmetry systems therequirements for photonic bandgaps are considerably relaxed compared to“complete” bandgaps in which all states, regardless of symmetry, areexcluded.

Accordingly, the dielectric stack reflector is highly reflective in thephotonic, bandgap because EM radiation cannot propagate through thestack. Similarly, the layers in confinement region 110 provideconfinement because they are highly reflective for incident rays in thebandgap. Strictly speaking, a photonic crystal is only completelyreflective in the bandgap when the index modulation in the photoniccrystal has an infinite extent. Otherwise, incident radiation, can“tunnel” through the photonic crystal via an evanescent mode thatcouples propagating modes on either side of the photonic crystal. Inpractice, however, the rate of such tunneling decreases exponentiallywith photonic crystal thickness (e.g., the number of alternatinglayers). It also decreases with the magnitude of the index-contrast inthe confinement region.

Furthermore, a photonic bandgap may extend over only a relatively smallregion of propagation vectors. For example, a dielectric stack may hehighly reflective for a normally incident ray and yet only partiallyreflective for an obliquely incident ray. A “complete photonic bandgap”is a bandgap that extends over all possible wavevectors and allpolarizations. Generally, a complete photonic bandgap is only associatedwith a photonic crystal having index modulations along three dimensions.However, in the context of EM radiation incident on a photonic crystalfrom an adjacent dielectric material, we can also define an“omidirectional photonic bandgap,” which is a photonic bandgap for allpossible wavevectors and polarizations for which the adjacent dielectricmaterial supports propagating EM modes, Equivalently, an omnidirectionalphotonic bandgap can be defined as a photonic hand gap for all EM modesabove the light line, wherein the light line defines the lowestfrequency propagating mode supported fey the material adjacent thephotonic crystal. For example, in air the light line is approximatelygiven by ω=cβ, where ω is the angular frequency of the radiation, β isthe wavevector, and c is the speed of light. A description of anomnidirectional planar reflector is disclosed in U.S. Pat. No.6,130,780, the contents of which are incorporated herein by reference.Furthermore, the use of alternating dielectric layers to provideomnidirectional reflection (in a planar limit) for a cylindricalwaveguide geometry is disclosed in U.S. Pat. No. 6,463,200, entitled“OMNIDIRECTIONAL MULTILAYER DEVICE FOR ENHANCED OPTICAL WAVEGUIDING” toYoel Fink et al., the contents of which are incorporated herein byreference.

When alternating the layers in confinement region 110 give rise to anomnidirectional bandgap with, respect to core 120, the guided modes arestrongly confined because, in principle, any EM radiation incident onthe confinement region from the core is completely reflected. However,such complete reflection only occurs when there are an infinite numberof layers. For a finite number of layers (e.g., about 10 opticalperiods), an omnidirectional photonic bandgap may correspond to areflection in a planar geometry of at least 95% for all angles ofincidence ranging from 0° to 80° and for all polarisations of EMradiation having frequency in the omnidirectional bandgap. Furthermore,even when photonic crystal fiber 100 has a confinement region with abandgap that is not omnidirectional, it may still support a stronglyguided mode, e.g., a mode with radiation losses of less than 0.1 dB/kmfor a range of frequencies in the bandgap. Generally, whether or not thebandgap is omnidirectional will depend on the size of the bandgapproduced by the alternating layer (which generally scales withindex-contrast of the two layers) and the lowest-index constituent ofthe photonic crystal.

In a Bragg-like configuration, the high-index layers may vary in indexand thickness, and/or the low-index layers may vary in index andthickness. The confinement region may also include a periodic structureincluding more than three layers per period (e.g., four or more layersper period). Alternatively, in some embodiments, the confinement regioncan include only two layers per period. Moreover, the refractive indexmodulation may vary continuously or discontinuously as a function offiber radius within the confinement region. In general, the confinementregion may be based on any index modulation that creates a photonicbandgap.

In the present embodiment, multilayer structure 110 forms a Braggreflector because it has a periodic index variation with respect to theradial axis. A suitable index variation is an approximate quarter-wavecondition. It is well-known that, for normal incidence, a maximum bandgap is obtained for a “quarter-wave” stack in which each, layer hasequal optical thickness λ/4, or equivalently d₈/d_(L)=n_(L)/n_(H), whered and n refer to the thickness and index, respectively, of thehigh-index and low-index layers in a fiber including two layers perperiod. Normal incidence corresponds to β=0. For a cylindricalwaveguide, the desired modes typically lie near the light line ω=cβ (inthe large core radius limit, the lowest-order modes are essentiallyplane waves propagating along z-axis, i.e., the waveguide axis). In thiscase, the quarter-wave condition becomes:

$\frac{d_{H}}{d_{L}} = \frac{\sqrt{n_{L}^{2} - 1}}{\sqrt{n_{H}^{2} - 1}}$

Strictly speaking, this equation may not be exactly optimal because thequarter-wave condition is modified by the cylindrical geometry, whichmay require the optical thickness of each layer to vary smoothly withits radial coordinate. Nonetheless, we find that this equation providesan excellent guideline for optimizing many desirable properties,especially for core radii larger than the mid-bandgap wavelength.

The radius of core 120 can vary depending on the end-use application offiber 120. The core radius can depend on the wavelength or wavelengthrange of the energy to be guided by the fiber, and on whether the fiberis a single or multimode fiber. For example, where the fiber is a singlemode fiber for guiding visible wavelengths (e.g., between about 400 nmand 800 nm) the core radius can be in the sub-micron to several micronrange (e.g., from about 0.5 μm to 5 μm). However, where the fiber is amultimode fiber for guiding IR wavelengths (e.g., from about 2 μm to 15μm, such as 10.6 μm), the core radius can be in the tens to thousands ofmicrons range (e.g., from about 10 μm to about 5,000 μm, such-as about500 μm to about 2,000 μm). The core radius can be greater than about 5λ.(e.g., more than about 10 λ, more than about 20 λ, more than about 30 λ,more than about 50 λ, more than about 100 λ), where a is the wavelengthof the guided energy.

As discussed previously, cladding 160 provides mechanical support forconfinement region 110. The thickness of cladding 160 can vary asdesired along major axis 161. The thickness of cladding 160 along minoraxis 162 can also vary but is generally less than the thickness alongthe major axis. In some embodiments, cladding 160 is substantiallythicker along the major axis than confinement region 110. For example,cladding 160 can be about 10 or more times thicker than confinementregion. 110 (e.g., more than about 20, more than about 30, more thanabout 50 times thicker) along the major axis.

The composition of cladding 160 is usually selected to provide thedesired mechanical support and protection for confinement region 110. Inmany embodiments, cladding 160 is formed from, materials that can beco-drawn with the confinement region 110. Criteria for selectingmaterials suitable for co-drawing are discussed, below. In someembodiments, cladding 160 can be formed from the same material(s) asused to form at least part of confinement region 110. For example, wherelayer 130 is formed from a polymer, cladding 160 can be formed from thesame polymer.

Turning now to the composition of layers 130, 140 and 150 in confinementregion 110, materials with a suitably high index of refraction to form ahigh index portion (e.g., layers 140 and 150) include chalcogenideglasses (e.g., glasses containing a chalcogen element, such as sulfur,selenium, and/or tellurium), heavy metal oxide glasses, amorphousalloys, and combinations thereof.

In addition to a chalcogen element, chalcogenide glasses may include oneor more of the following elements: boron, aluminum, silicon, phosphorus,sulfur, gallium, germanium, arsenic, indium, tin, antimony, thallium,lead, bismuth, cadmium, lanthanum and the halides (fluorine, chlorine,bromide, iodine).

Chalcogenide glasses can be binary or ternary glasses, e.g., As—S,As—Se, Ge—S, Ge—Se, As—Te, Sb—Se, As—S—Se, S—Se—Te, As—Se—Ta, As—Te,Ge—S—Te, Ge—Se—Te, Ge—S—Se, As—Ge—Se, As—Ge—Te, As—Se—Pb, As—S—Tl,As—Se—Tl, As—Te—Tl, As—Se—Ga, Ga—La—S, Ge—Sb—Se or complex,multi-component glasses based on these elements such as As—Ga—Ge—S,Pb—Ga—Ge—S, etc. The ratio of each element in a chalcogenide glass canbe varied. For example, a chalcogenide glass with a suitably highrefractive index may be formed, with 5-30 mole % Arsenic, 20-40 mole %Germanium, and 30-60 mole % Selenium. As another example, As₂Se₃ can beused.

Examples of heavy metal oxide glasses with high refractive indicesinclude Bi₂O₃-, PbO-, Tl₂O₃-, Ta₂O₃-, TiO₂-, and TeO₂-containingglasses.

Amorphous alloys with suitably high indices of refraction include Al—Te,R—Te(Se) (R=alkali).

Suitable materials for high and low index layers can include inorganicmaterials such as inorganic glasses or amorphous alloys. Examples ofinorganic glasses include oxide glasses (e.g., heavy metal oxideglasses), halide glasses and/or chalcogenide glasses, and organicmaterials, such as polymers. Examples of polymers includeacrylonitrile-butadienc-styrene (ABS), poly methylmethacrylate (PMMA),cellulose acetate butyrate (CAB), polycarbonates (PC), polystyrenes (PS)(including, e.g., copolymers styrene-butadiene (SBC),methylestyrene-acrylonitrile, styrene-xylylene, styrene-ethylene,styrene-propylene, styrene-acylonitrile (SAN)), polyetherimide (PES),polyvinyl acetate (PVAC), polyvinyl alcohol (PVA), polyvinyl chloride(PVC), pelyoxymethylene; polyformaldehyde (polyacetal) (POM), ethylenevinyl acetate copolymer (EVAC), polyamide (PA), polyethyleneterephthalate (PBTP), fluoropolymers (including, e.g.,polytetrafluoroethylene (PTFE), polyperfluoroalkoxythylene (PFA),fluorinated ethylene propylene (FEP)), polybutylene terephthalate(PBTP), low density polyethylene (PB), polypropylene (PP), poly methylpentenes (PMP) (and other polyolefins, including cyclic polyoleflns),polytetrafluoroethylene (PTFE), polysulfides (including, e.g.,polyphenylene sulfide (PPS), and polysulfones (including, e.g.,polysulfone (PSU), polyehtersulfone (PES), polyphenylsulpbone (PPSU),polyarylalkylsufone, and polysulfonates). Polymers can be homopolymersor copolymers (e.g., (Co)poly(acrylamide-acrylonltrile) and/oracrylonitrile stytene copolymers). Polymers can include polymer blends,such as blends of polyamides-polyolefins, polyamides-polycarbonates,and/or PBS-polyolefins, for example.

Further examples of polymers that can be used include cyclic olefinpolymers (COPs) and cyclic olefin copolymers (COGs). In someembodiments, COPs and COGs can be prepared by polymerizing norbomenmonomers or copolymerization norbomen monomers and other polyolefins(polyethylene, polypropylene). Commercially-available COPs and/or COCscan he used, including, for example, Zeonex® polymers (e.g., Zeonex®E48R) and Zeonor® copolymers (e.g., Zeonor® 1600), both available fromZeon Chemicals L.P. (Louisville, Ky.). COCs can also be obtained fromPromerus LLC (Brecksville, Ohio) (e.g., such as FS1700).

Suitable oxide glasses may include glasses that contain one or more ofthe following compounds: 0-40 mole % of M₂O where M is Li, Na, K, Rb, orCs; 0-40 mole % of M′O where M′ is Mg, Ca, Sr, Ba, Zn, or Pb; 0-40 mole% of M″₂O₃ where M″ is B, Al, Ga, In, Sn, or Bi; 0-60 mole % P₂O₅; and0-40 mole % SiO₂.

Portions of photonic crystal fiber waveguides (e.g., layers inconfinement region 110) can optionally include other materials. Forexample, any portion can include one or more materials that change theindex of refraction of the portion. A portion can include a materialthat increases the refractive index of the portion. Such materialsinclude, for example, germanium oxide, which can increase the refractiveindex of a portion containing a bore-silicate glass. Alternatively, aportion can include a material that decreases the refractive index ofthe portion. For example, heron oxide can decrease the refractive indexof a portion containing a borosilicate glass.

Portions of photonic crystal fiber waveguides can he homogeneous orinhomogeneous. For example, one or more portions can includenano-particles (e.g., particles sufficiently small to minimally scatterlight at guided wavelengths) of one material embedded in a host materialto form an inhomogeneons portion. An example of this is a high-indexpolymer composite formed, by embedding a high-index chalcogenide glassnano-particles in a polymer host. Further examples include CdSe and orPbSe nano-particles in an inorganic glass matrix.

Portions of photonic crystal fiber waveguides can include materials thatalter the mechanical, rheological and/or thermodynamic behavior of thoseportions of the fiber. For example, one or more of the portions caninclude a plasticizer. Portions may include materials that suppresscrystallization, or other undesirable phase behavior within the fiber.For example, crystallization in polymers may he suppressed by includinga cross-linking agent (e.g., a photosensitive cross-linking agent). Inother examples, if a glass-ceramic material was desired, a nucleatingagent, such as TiO₂ or ZrO₂, can be included in the material.

Portions can also include compounds designed to affect the interfacebetween adjacent portions in the fiber (e.g., between the low index andhigh index layers). Such compounds include adhesion promoters andcompatibilizers. For example, an organo-silane compound can he used topromote adhesion between a silica-based glass portion and a polymerportion. For example, phosphorus or P₂O₅ is compatible with bothchalcogenide and oxide glasses, and may promote adhesion betweenportions formed from these glasses.

Fiber waveguides can include additional materials specific to particularfiber waveguide applications. In fiber amplifiers, for example, any ofthe portions can be formed of any dopant or combination of dopantscapable of interacting with an optical signal in the fiber to enhanceabsorption or emission of one or more wavelengths of light by the fiber,e.g., at least one rare earth ion, such as erbium ions, ytterbium ionsneodymium ions, holmium ions, dysprosium ions, and/or thulium ions.

Portions of high index-contrast waveguides can include one or morenonlinear materials. Nonlinear materials are materials that enhance thenonlinear response of the waveguide. In particular, nonlinear materialshave a larger nonlinear response than silica. For example, nonlinearmaterials have a Kerr nonlinear index, n⁽²⁾, larger than the Kerrnonlinear index of silica (i.e., greater than 3.5×10⁻²⁰ m/W, such asgreater than 5×10⁻²⁰ m²/W, greater than 10×10 ⁻²⁰ m²/W, greater than20×10⁻²⁰ m²/W, greater than 100×10⁻²⁰ m²/W, greater than 200×10⁻²⁰m²/W).

When making a robust fiber waveguides using a drawing process, not everycombination of materials with desired optical properties is necessarilysuitable. Typically, one should select materials that are theologically,thermo-mechanically, and physico-chemically compatible. Several criteriafor selecting compatible materials will now be discussed.

A first criterion is to select materials that are theologicallycompatible. In other words, one should select materials that havesimilar viscosities over a broad temperature range, corresponding to thetemperatures experience during the different stages of fiber drawing andoperation. Viscosity is the resistance of a fluid to flow under anapplied shear stress. Here, viscosities are quoted in units of Poise.Before elaborating on rheological compatibility, it is useful define aset of characteristic temperatures for a given material, which aretemperatures at which the given material has a specific viscosity.

The annealing point, T_(a), is the temperature at which a material has aviscosity 10¹³ Poise. T_(a) can be measured using a Model SP-2A Systemfro. Orion Ceramic Foundation (Westerville, Ohio). Typically T_(a) isthe temperature at which the viscosity of a piece of glass is low enoughto allow for relief of residual stresses.

The softening point, T_(s), is the temperature at which a material has aviscosity 10^(7.65) Poise, T_(s) can be measured using a softening pointinstrument, e.g., Model SP-3A from Orton Ceramic Foundation(Westerville, Ohio). The softening point is related to the temperatureat which the materials flow changes from plastic to viscous in nature.

The working point, T_(w), is the temperature at which a material has aviscosity 10⁴ Poise. T_(w) can be measured using a glass viscometer,e.g., Model SP-4A from Orton Ceramic Foundation (Westervilie, Ohio). Theworking point is related to the temperature at which a glass can beeasily drawn into a fiber. In some embodiments, for example, where thematerial is an inorganic glass, the material's working point temperaturecan be greater than 250° C., such as about 300° C., 400° C., 500° C. ormore.

The melting point, T_(m), is the temperature at which a material has aviscosity 10² Poise. T_(m) can also be measured using a glassviscometer, e.g., Model SP-4A from Orton Ceramic Foundation(Westerville, Ohio), The melting point is related to the temperature atwhich a glass becomes a liquid and control of the fiber drawing processwith respect to geometrical maintenance of the fiber becomes verydifficult

To be rheologically compatible, two materials should have similarviscosities over a broad temperature range, e.g., from the temperatureat which the fiber is drawn down to the temperature at winch the fibercan no longer release stress at a discernible rates (e.g., at T_(a)) orlower. Accordingly, the working temperature of two compatible materialsshould be similar, so that the two materials How at similar rates whendrawn. For example, if one measures the viscosity of the first material,η₁(T) at the working temperature of the second material, T_(w2),η₁(T_(w2)) should he at least 10³ Poise, e.g., 10⁴ Poise or 10⁵ Posse,and no more than 10⁶ Poise. Moreover, as the drawn; fiber cools thebehavior of both materials should change front viscous to elastic atsimilar temperatures. In Oliver words, the softening temperature of thetwo materials should be similar. For example, at the softeningtemperature of the second material, T_(s2), the viscosity of the firstmaterial, η₁(T_(s2)) should be at least 10⁶ Poise, e.g., 10⁷ Poise or10⁸ Poise and no more than 10⁹ Poise. In preferred embodiments, itshould be possible to anneal both materials together, so at theannealing temperature of the second material, T_(a2), the viscosity ofthe first material η₁(T_(a2)) should be at least 10⁸ Poise (e.g., atleast 10⁶ Poise, at least 10¹⁰ Poise, at least 10¹¹ Poise, at least 10¹²Poise, at least 10¹³ Poise, at least 10¹⁴ Poise).

Additionally, to be rheologically compatible, the change in viscosity asa function of temperature (i.e., the viscosity slope) for bothmaterials, should preferably match as close as possible.

A second selection criterion is that the thermal expansion coefficients(TEC) of each material should be similar at temperatures between theannealing temperatures and room temperature. In other words, as thefiber cools and its rheology changes from liquid-like to solid-like,both materials' volume should change by similar amounts. If the twomaterials TEC's are not sufficiently matched, a large differentialvolume change between two fiber portions can result in a large amount ofresidual stress buildup, which can cause one or more portions to crackand/or delaminate. Residual stress may also cause delayed fracture evenat stresses well below the material's fracture stress.

The TEC is a measure of the fractional change in sample length with achange in temperature. This parameter can he calculated for a givenmaterial from the slope of a temperature-length (or equivalently,temperature-volume) curve. The temperature-length curve of a materialcan be measured using e.g., a dilatometer, such, as a Model 1200Ddilatometer from Orton Ceramic Foundation (Westerville, Ohio). The TECcan be measured either over a chosen temperature range or as theinstantaneous change at a given temperature. This quantity has the units° C.⁻¹.

For many materials, there are two linear regions in thetemperature-length curve that have different slopes. There is atransition region where the curve changes from the first to the secondlinear region. This region is associated with a glass transition, wherethe behavior of a glass sample transitions from that normally associatedwith a solid material to that normally associated with a viscous fluid.This is a continuous transition and is characterized by a gradual changein the slope of the temperature-volume curve as opposed to adiscontinuous change in slope. A glass transition temperature, T_(g),can be defined as the temperature at which the extrapolated glass solidand viscous fluid lines intersect. The glass transition, temperature isa temperature associated with a change in the materials rheology from abrittle solid to a solid that can flow. Physically, the glass transitiontemperature is related to the thermal energy required, to excite variousmolecular translational and rotational modes in the material. The glasstransition temperature is often taken as the approximate annealingpoint, where the viscosity is 10¹³ Poise, but in fact, the measuredT_(g) is a relative value and is dependent upon the measurementtechnique.

A dilatometer can also be used to measure a dilatometric softeningpoint, T_(ds). A dilatometer works by exerting a small compressive loadon a sample and heating the sample. When the sample temperature becomessufficiently high, the material starts to soften and the compressiveload causes a deflection in the sample, when is observed as a decreasein volume or length. This relative value is called the dilalometricsoftening point and usually occurs when the materials viscosity isbetween 10¹⁰ and 10^(12.5) Poise. The exact T_(ds) value for a materialis usually dependent upon the instrument and measurement parameters.When similar instruments and measurement parameters are used, thistemperature provides a useful measure of different materials theologicalcompatibility in this viscosity regime.

As mentioned above, matching the TEC is an important consideration forobtaining fiber that is tree from excessive residual stress, which candevelop in the fiber during the draw process. Typically, when the TEC'sof the two materials are not sufficiently matched, residual stressarises as elastic stress. The elastic stress component stems from thedifference in volume contraction between different materials in thefiber as it cools from the glass transition temperature to roomtemperature (e.g., 25° C.). The volume change is determined by the TECand the change in temperature. For embodiments in which the materials inthe fiber become fused or bonded at any interface during the drawprocess, a difference in their respective TEC's will result in stress atthe interface. One material will be in tension (positive stress) and theother in compression (negative stress), so that the total stress iszero. Moderate compressive stresses themselves are not usually a majorconcern for glass fibers, but tensile stresses are undesirable and maylead to failure over time. Hence, it is desirable to minimize thedifference in TEC's of component materials to minimise elastic stressgeneration in a fiber during drawing, for example, in a composite fiberformed from two different materials, the absolute difference between,the TEC's of each glass between T_(g) and room temperature measured witha dilatometer with a heating rate of 3° C./min, should be no more than5×10⁻⁶ ° C.⁻¹ (e.g., no more than 4×10⁻⁶ ° C.⁻¹, no more than 3×10⁻⁶ °C.⁻¹, no more than 2×10⁻⁶ ° C.⁻¹, no more than 1×10⁻⁶ ° C.⁻¹, no morethan 5×10⁻⁷ ° C.⁻¹, no more than 4×10⁻⁷ ° C.⁻¹, no more than 3×10⁻⁷ °C.⁻¹, no more than 2×10⁻⁷ ° C.⁻¹).

While selecting materials having similar TEC's can minimize an. elasticstress component, residual stress can also develop from viseoelasticstress components. A viseoelastic stress component arises when there issufficient difference between strain point or glass transitiontemperatures of the component materials. As a material cools below T_(g)it undergoes a sizeable volume contraction. As the viscosity changes inthis transition upon cooling, the time needed to relax stress increasesfrom aero (instantaneous) to minutes. For example, consider a compositepreform made of a glass and a polymer having different glass transitionranges (and different T_(g)'s). During initial drawing, the glass andpolymer behave as viscous fluids and stresses due to drawing strain arerelaxed instantly. After leaving the hottest part of the draw furnace,the fiber rapidly loses heat, causing the viscosities of the fibermaterials to increase exponentially, along with the stress relaxationtime. Upon cooling to its T_(g), the glass and polymer cannotpractically release any more stress since the stress relaxation time hasbecome very large compared with the draw rate. So, assuming thecomponent materials possess different T_(g) values, the first materialto cool to its T_(g) can no longer reduce stress, while the secondmaterial is still above its T_(g) and can release stress developedbetween the materials. Once the second material cools to its T_(g),stresses that arise between the materials can no longer be effectivelyrelaxed. Moreover, at this point the volume contraction of the secondglass is much greater than the volume contraction of the first material(which is now below its T_(g) and behaving as a brittle solid). Such asituation can result sufficient stress buildup between the glass andpolymer so that one or both of the portions mechanically fail. Thisleads us to a third selection criterion for choosing fiber materials: itis desirable to minimize the difference in T_(g)'s of componentmaterials to minimise viseoelastic stress generation in a fiber duringdrawing. Preferably, the glass transition temperature of a firstmaterial, T_(g1), should, be within 100° C. of the glass transitiontemperature of a second material, T_(g2) (e.g., |T_(g1)−T_(g2)| shouldbe less than 90° C., less than 80° C., less than 70° C., less than 60°C., less than 50° C., less than 40° C., less than 30° C., less than 20°C., less than 10° C.).

Since there are two mechanisms (i.e., elastic and viseoelastic) todevelop permanent stress in drawn fibers due to differences betweenconstituent, materials, these mechanisms may be employed to offset oneanother. For example, materials constituting a fiber may naturallyoffset the stress caused by thermal expansion mismatch if mismatch inthe materials, T_(g)'s results in stress of the opposite sign.Conversely; a greater difference in T_(g) between materials isacceptable if the materials' thermal expansion, will reduce the overallpermanent stress. One way to assess the combined effect of thermalexpansion and glass transition temperature difference is to compare eachcomponent-materials' temperature-length curve. After finding T_(g) foreach material using the foregoing slope-tangent method, one of thecurves is displaced along the ordinate axis such that the curvescoincide at the tower T_(g) temperature value. The difference in y-axisintercepts at room temperature yields the strain, ε, expected if theglasses were not conjoined. The expected tensile stress, σ, for thematerial showing the greater amount of contraction over the temperaturerange from T_(g) to room temperature, can be computed, simply from thefollowing equation:

σ=E·ε,

where E is the elastic modulus for that material. Typically, residualstress values less than 100 MPa (e.g., less than 50 MPa, less than 30MPa), are sufficiently small to indicate that two materials arecompatible.

A fourth selection criterion is to match the thermal stability ofcandidate materials. A measure of the thermal stability is given by thetemperature interval (T_(x)−T_(g)), where T_(x) is the temperature atthe onset of the crystallisation, as a material cools slowly enough thateach molecule can find its lowest energy state. Accordingly, acrystalline phase is a more energetically favorable state for a materialthan a glassy phase. However, a material's glassy phase typically hasperformance and/or manufacturing advantages over the crystalline phasewhen it comes to fiber waveguide applications. The closer thecrystallization temperature is to the glass transition temperature, themore likely the material is to crystallise during drawing, which can bedetrimental to the fiber (e.g., by introducing optical inhomogeneitiesinto the fiber, which can increase transmission losses). Usually athermal stability interval, (T_(x)−T_(g)) of at least about 80° C.(e.g., at least about 100° C.) is sufficient to permit fiberization of amaterial by drawing fiber from a preform. In preferred embodiments, thethermal stability interval is at least about 120° C., such as about 150°C. or more, such as about 200° C. or more. T_(x) can be measured using athermal analysis instrument, such as a differential thermal analyser(DTA) or a differential scanning calorimeter (DSC).

A further consideration when selecting materials that can be co-drawnare the materials' melting temperatures, T_(m). At the meltingtemperature, the viscosity of the material becomes too low tosuccessfully maintain precise geometries during the fiber draw process.Accordingly, in preferred embodiments the melting temperature of onematerial is higher than the working temperature of a second,rheologlcally compatible material in other words, when heating apreform, the preterm reaches a temperature at it can be successfullydrawn before either material in the preform melts.

One example of a pair of materials which can be co-drawn and whichprovide a photonic crystal fiber waveguide with high index contrastbetween layers of the confinement region are As₂Se₃ and the polymer PES,As₂Se₃ has a glass transition temperature (T_(g)) of about 180° C. and athermal expansion coefficient (TEC) of about 24×10⁻⁶/° C. At 10.6 μm,As₂Se₃ has a refractive index of 2.7775, as measured by Hartouni andcoworkers and described in Proc. SPIE, 505, 11 (1984), and an absorptioncoefficient, α, of 5.8 dB/m, as measured by Voigt and Linke anddescribed in “Physics and Applications of Non-Crystalline Semiconductorsin Optoelectronics,” Ed. A. Andriesh and M. Bertolotti, NATO ASI Series,3. High Technology, Vol. 36, p. 155 (1996). Both of these references arehereby Incorporated by reference in their entirety. PES has a TEC ofabout 55×10⁻⁶/° C. and has a refractive index of about 1.65.

In some embodiments, photonic crystal fibers, such as fiber 100, can bemade by rolling a planar multilayer article into a spiral structure anddrawing a fiber from a preform derived from the spiral structure.

Referring to FIG. 2A, to prepare a preform, one or more glasses aredeposited 220 on opposing surfaces 211 and 212 of a polymer film 210.The glass can be deposited by methods including thermal evaporation,chemical vapor deposition, or sputtering. Referring to FIG. 2B, thedeposition process provides a multilayer article 240 composed of layers230 and 231 of glass on polymer film 210.

Referring, to FIG. 2C, following the deposition step, multilayer film240 is roiled around a mandrel 255 (e.g., a hollow glass, such as aborosilicate glass, or polymer tube) to form a spiral tube. A number(e.g., about three to ten) of polymer films are then wrapped around thespiral tube to form a preform wrap. In some embodiments, the polymerfilms are made from the same polymer or glass used to form multilayerarticle. Under vacuum, the preform wrap is heated to a temperature abovethe glass transition temperature of the polymer(s) and glass(es) formingmultilayer film 240 and the films wrapped around the spiral tube. Thepreform wrap is heated for sufficient time for the layers of the spiraltube to fuse to each other and for the spiral tube to fuse to polymerfilms wrapped around it. The temperature and length of time of heatingdepends on the preform wrap composition. Where the multilayer iscomposed of As₂Se₃ and PES and the wrapping films are composed of PES,for example, heating for 15-20 minutes (e.g., about 18 minutes) at about200-300° C. (e.g., about 250° C.) is typically sufficient. The heatingfuses the various layers to each other, consolidating the spiral tubeand wrapping films. The consolidated structure is shown in FIG. 2D. Thespiral tube consolidates to a multilayer region 200 corresponding torolled multilayer film 240, The wrapped polymer films consolidate to amonolithic support cladding 270, The consolidated structure retains ahollow core 250 of mandrel 255.

As an alternative to wrapping polymer films around the spiral tube toprovide support cladding 270, the spiral tube can be inserted into ahollow tube with inner diameter matching the outer diameter of thespiral tube.

Referring to FIG. 2E, portions at opposing sides 271 and 272 of cladding270 are removed to provide a perform 280 having an asymmetriccross-sectional shape. The portions can be removed by cutting, orshaving the perform (e.g., with a blade or milling cutter) or bygrinding (e.g., with a grinding wheel) and polishing the sides of theperform.

Mandrel 255 is removed from the consolidated structure to provide ahollow preform that is then drawn into a fiber. The preform has the samecomposition and relative dimensions (e.g., core radius to thickness oflayers in the confinement region) of the final fiber. The absolutedimensions of the fiber depend on the draw ratio used. Long lengths offiber can be drawn (e.g., up to thousands of meters). The drawn fibercan then be cut to the desired length.

Preferably, consolidation occurs at temperatures below the glasstransition for the mandrel so that the mandrel provides a rigid supportfor the spiral tube. This ensures that the multilayer film does .notcollapse on itself under the vacuum. The mandrel's composition can feeselected so that it releases from the innermost layer of the multilayertube after consolidation. Alternatively, where the mandrel adheres tothe innermost layer of the multilayer tube during consolidation, it canbe removed chemically, e.g., by etching. For example, in embodimentswhere the mandrel is a glass capillary tube, it can be etched, e.g.,using hydrofluoric acid, to yield the preform.

In embodiments where a solid core is desired, the multilayer tube can beconsolidated around a solid mandrel that is co-drawn with the otherparts of lire fiber. Alternatively, in other embodiments, the multilayerfilm can be rolled without a mandrel to provide a self-supporting spiraltube.

In some embodiments, the fiber asymmetry can be introduced after thefiber is drawn from a perform. For example, a fiber can be shaved orground as part of the production process after being drawn but beforebeing spooled. In certain embodiments, the preform can be shaved, andthen the fiber can he shaved further after if has been drawn.

Photonic crystal fiber waveguides prepared using the previouslydiscussed technique can be made with a low defect density. For example,waveguides can have less than about one defect per 5 meters of fiber(e.g., less than about, one defect per 10 meters, 20 meters, 50 meters,100 meters of fiber), Defects include both material defects (e.g.,impurities) and structural defects (e.g.,, delamination between layers,cracks with layers), both of which can scatter guided radiation from thecore resulting in signal, loss and can cause local heating of the fiber.Accordingly, reducing fiber defects is desirable in applicationssensitive to signal loss (e.g., in high power applications whereradiation absorbed, by the fiber can cause damage to the fiber).

In fiber 100, only the outer surface of the cladding has an asymmetriccross-section. More generally, however, other portions of photoniccrystal fibers can have an asymmetric cross-section. In certain,embodiments, the confinement region and/or core can also have anasymmetric cross-section. For example, referring to FIG. 3, a photoniccrystal fiber 300 has a confinement region 310 having an asymmetriccross-section surrounded by an asymmetric cladding 330. The core 320 offiber 300 also has an asymmetric cross-section. Confinement region 310,core 320, and cladding 330 have an elliptical cross-sectional shape. Theconfinement region includes an inner seam 315 that is located on themajor axis of the ellipse, although, more generally, the inner seam canhe located at other orientations with respect to the elliptical axes.

In some embodiments, the asymmetry of the core anchor confinement regioncan affect the guiding properties of the fiber. For example, in certainembodiments, asymmetric fibers can maintain the polarization state ofguided radiation (i.e., can be polarisation maintaining fibers).

Cladding 330 and core 320 have dimensions b₃₃₀ and b₃₂₀, respectively,along the major axis. Correspondingly, cladding 330 and core 320 havedimensions a₃₃₀ and a₃₂₀ along the minor axis. Respective ellipticitiesfor the cladding and core can be expressed mathematically as:

${ɛ_{330} = \frac{\left( {b_{330} - a_{330}} \right)}{\frac{1}{2}\left( {b_{330} - a_{330}} \right)}},{and}$$ɛ_{320} = {\frac{\left( {b_{320} - a_{320}} \right)}{\frac{1}{2}\left( {b_{320} - a_{320}} \right)}.}$

In general, ε₃₃₀ and ε₃₂₀ can he the same or different. In someembodiments, □₃₂₀>ε₃₃₀. For example, in embodiments where an asymmetriccore is desired, such as in a polarization-maintaining fiber, a highcore ellipticity may be desired. Alternatively, in other embodiments,ε₃₃₀>ε₃₂₀.

Fiber 300 can be made by applying a force (e.g., a compressive force) toopposing sides of the fiber or fiber perform. The force can be appliedwhile the fiber or perform is at an elevated temperature (e.g., at atemperature where components of the fiber have a softened) to facilitatethe deformation. The deformation sets once the fiber or fiber performcools.

Furthermore, while fiber 300 has an elliptical cross-sectional, shape,and fiber 100 has a shape composed of two circular arcs and two straightlines, in general, fibers can have other shapes. For example, fibers canhave asymmetric polygonal shapes, can be formed from arcuate portionshaving different radii of curvature, and/or from arcuate portions thatcurve in opposite directions. Generally, the shape should provide thefiber with a preferred bending plane.

While the foregoing fibers are asymmetric with respect to theircross-sectional shape, in general, fibers can be asymmetric in a varietyof ways in order to provide a preferred bend plane. For example, in someembodiments, fibers can include material asymmetries that give rise to apreferred bend plane. Material asymmetries refer to variations betweenthe material properties of different portions of a fiber that cause thefiber to bend preferably in a particular way. For example, a portion ofa fiber cladding can be formed from a material that is mechanically lessrigid that other portions, causing the fiber to bend preferably at thatportion. Mechanical variations can be caused by compositional changes orby physical differences in portions having the same composition.Compositional differences can be introduced, e.g., by doping portions ofa fiber or fiber-preform with a dopant, that alters the mechanicalproperties of a fiber. As another example, compositional differences canhe introduced by forming different portions of a fiber from differentcompounds. Physical differences refer to, e.g., differences in thedegree of crystallinity in different portions of a fiber. Physicaldifferences, such as differences in crystallinity, can be introduced byselectively heating and/or cooling portions of a fiber during fiberfabrication, and/or using different, rates of heating/cooling ondifferent fiber portions.

Referring to FIG. 4A, another example of a photonic crystal 400 includesa confinement region 410 surrounding a core 420, and a cladding 430surrounding confinement region 410. Confinement region 410 includes aseam 415. Cladding 430 includes two portions 431 and 432 that arecomposed of different materials than the rest of the cladding. Forexample, in some embodiments, portions 431 and 432 are formed from amaterial that has a higher mechanical stiffness than the rest ofcladding 430. As an example, portions 431 and 432 can be formed from apolymer that has a higher density of cross-linking than a polymerforming the rest of cladding 430. In embodiments where portions 431 and433 are stiffer than the rest of the cladding, fiber 400 includes apreferred bend plane that is perpendicular to the plane that includes adiameter that intersects portions 431 and 432. Alternatively, in certainembodiments, portions 431 and 432 are formed from materials that areless stiff than the material forming the rest of cladding 430. In snobcases, fiber 400 has a preferred bend plane that corresponds to theplane that includes a diameter that intersects portions 431 and 432.Portions 431 and 432 can ran substantially along the entire length offiber 400, or just along segments of the fiber.

Referring to FIG. 4B, a further example of a photonic crystal fiber 440includes a confinement region 450 surrounding a core 460. A cladding 470surrounds confinement region 450. Confinement region 460 includes a seam455. Embedded in cladding 470 are two stiffening elements 451 and 452,which are formed from materials having higher mechanical stiffness thanthe rest of cladding 470. For example, stiffening elements 451 and 452can be formed from wires (e.g., steel wires) that are inserted intoholes that run the length of fiber 440. As an example, forming photoniccrystal fiber can include machining grooves into the preform cladding onopposite sides of the preform core. The fiber is drawn at a temperatureand tension such that the grooves are preserved in the drawn fiber.Finally, wires are inserted into the grooves and an adhesive (e.g., anepoxy) is used to fix the wires in place.

Stiffening elements 451 and 452 create a bend plane orthogonal to thediagonal connecting the two stiffening elements. Other embodiments caninclude more than two stiffening elements. Further, in certainembodiments, claddings can included embedded elements that are of lowermechanical stiffness than the rest of the cladding, providing a bendplane in the same plane as the diagonal intersecting the elements. Forexample, a cladding can include two or more holes that run along thelength of the fiber.

In some embodiments, asymmetry can be introduced on one side of thefiber only, rather than on opposing sides as for the embodimentsdescribed above. For example, referring to FIG. 4C, a photonic crystalfiber 470 includes a confinement region 480 that surrounds a core 490.Confinement region 480 includes a seam 485. Confinement region 480 issurrounded by a cladding 495, that, in cross-section, includes acircular portion 496 and a flat portion 497. Flat portion 497 can beformed by shaving or grinding the fiber or fiber preform on one sideonly. Flat portion 497 is positioned so that seam 485 lies between itand core 490 in a radial direction from the fiber axis. Fiber 470 has abend plane that intersects portion 497. Moreover, fiber 470 bendspreferably so that seam 385 is on the inside of the bond.

In some embodiments, fibers can include a symmetric first cladding, butcan include additional structure outside of the cladding that cause thefiber to bend preferably in a particular plane. For example, fibers canbe placed in one or more jackets that are asymmetric when it comes toallowing the fiber to bend. Referring to FIG. 4D, for example, aphotonic crystal fiber includes a jacket 4050 that surrounds a cladding4040. Cladding 4040, in turn, surrounds, a confinement region 4030,which surrounds a core 4020. Confinement region 4030 includes a seam4010. Jacket 4050 has an elliptical cross-section that provides a bendplane. Cladding 4040 has a circular cross-section. In certainembodiments, cladding 4040, confinement region 4030, and core 4020 areformed by drawing a preform structure. The drawn fiber is then insertedinto a hole in jacket 4050. The orientation of the jacket with respect,to the rest of the fiber can be secured by, for example, applying anadhesive to the interface of the jacket and the cladding, or byconsolidating the jacket, onto the cladding (e.g., using heat).

Although the fibers described, above include confinement regions thathave a seam, in general, embodiments of fibers with a bend plane can bedesigned to position other features in or away from the bend plane. Forexample, in some embodiments, fibers can include extended defects (e.g.,structural or compositional defects) other than a seam that is desirablypositioned away from the bend plane. Moreover, embodiments can includeconfinement regions with no seams (e.g., confinement regions that areformed from a number of annular layers).

A number of embodiments of photonic crystal fibers have been described.However, further embodiments can include other types of photonic crystalfibers. For example, while the foregoing description relates to photoniccrystal fibers having spiral confinement regions, the principlesdescribed herein can be applied to non-spiral photonic crystal fibers.In general, these principles can be applied to fibers composed of aconfinement region with one or more concentric layers. Embodiments ofphotonic crystal fibers are described in the following patents, patentapplications, and provisional patent applications: U.S. Pat. No.6,625,364, entitled “LOW-LOSS PHOTONIC CRYSTAL WAVEGUIDE HAVING LARGECORE RADIUS;”U.S. Pat. No. 6,563,98.1, entitled “ELECTROMAGNETIC MODECONVERSION IN PHOTONIC CRYSTAL MULTIMODE WAVEGUIDES;” U.S. patentapplication Ser. No. 10/057,440, entitled “PHOTONIC CRYSTAL OPTICALWAVEGUIDES HAVING TAILORED DISPERSION PROFILES,” and filed on Jan. 25,2002; U.S. patent application Ser. No. 10/121,452, entitled “HIGHINDEX-CONTRAST FIBER WAVEGUIDES AMD APPLICATIONS,” and filed on Apr. 12,2002; U.S. Pat. No. 6,463,200, entitled “OMNIDIRECTIONAL MULTILAYERDEVICE FOR ENHANCED OPTICAL WAVEGUIDING;” Provisional 60/428,382,entitled “HIGH POWER/WAVEGUIDE,” and filed on Nov. 22, 2002; U.S. patentapplication Ser. No. 10/196,403, entitled “METHOD OF FORMING REFLECTINGDIELECTRIC MIRRORS,” and filed on Jul. 16, 2002; U.S. patent applicationSer. No. 10/720,606, entitled “DIELECTRIC WAVEGUIDE AND METHOD OP MAKINGTHE SAME,” and filed on Nov. 24, 2003; U.S. patent application Ser. No.10/733,873, entitled “FIBER WAVEGUIDES AND METHODS OF MAKING SAME,” andfiled on Dec. 10, 2003; Provisional Patent Application No. 60/603,067,entitled “PHOTONIC CRYSTAL WAVEGUIDES AND SYSTEMS USING SUCHWAVEGUIDES,” and filed on Aug. 20, 2004. The contents of each, of theabove mentioned patents, patent applications, and provisional patentapplications are hereby incorporated by reference in their entirety.

Moreover, while the foregoing embodiments pertain to photonic crystalfibers having solid confinement regions, photonic crystal fibers canalso include confinement regions with portions that are not solid, suchas holey fibers.

The photonic crystal fibers described herein may be used in a variety ofapplications. For example, the photonic crystal fibers can be used inmedical laser systems. Referring to FIG. 5, an example of a medicallaser system 500 includes a CO₂ laser 510, and a photonic crystal fiber520 having a hollow core to guide radiation 512 from the laser to atarget location 99 of a patient. Radiation 512 has a wavelength of 10.6microns. Laser radiation 512 is coupled by a coupling assembly 530 intothe hollow core of photonic crystal fiber 520, which delivers theradiation through a handpiece 540 to target location 599. During use, anoperator (e.g., a medical practitioner, such as a surgeon, a dentist, anophthalmologist, or a veterinarian) grips a portion. 542 of handpiece540, and manipulates the handpiece to direct laser radiation 513 emittedfrom an output end of photonic crystal fiber 520 to target location 599in order to perform a therapeutic function at the target location. Forexample, the radiation can be used to excise, incise, ablate, orvaporise tissue at the target location.

CO₂ laser 510 is controlled by an electronic controller 550 for settingand displaying operating parameters of the system. The operator controlsdelivery of the laser radiation using a remote control 552, such as afoot pedal. In some embodiments, the remote control is a component ofhandpiece 540, allowing the operator to control the direction of emittedlaser radiation and delivery of the laser radiation with one hand orboth hands.

In addition to grip portion 542, handpiece 540 includes a stand off tip544, which maintains a desired distance (e.g., from about 0.1millimeters to about 30 millimeters) between the output end of fiber 520and target tissue 599. The stand off tip assist the operator inpositioning the output end of photonic crystal fiber 520 from targetlocation 599, and can also reduce clogging of the output end due todebris at the target location. In some embodiments, handpiece 540includes optical components (e.g., a lens or lenses), which focus thebeam emitted from the fiber to a desired spot size. The waist of thefocused beam can be located at or near the distal end of the stand offtip.

In some embodiments, fiber 520 can be easily installed and removedfrom-coupling-assembly 530, and from handpiece 540 (e.g., usingconventional fiber optic connectors). This can facilitate ease of use ofthe system in single-use applications, where the fiber is replaced aftereach procedure.

Typically, CO₂ laser 510 has an average output power of about 5 Watts toabout 80 Watts at 10.6 microns (e.g., about 10 Watts or more, about 20Watts or more). In many applications, laser powers of about 5 Watts toabout 30 Watts are sufficient for the system to perform its intendedfunction. For example, where system 500 is being used to excise orincise tissue, the radiation is confined to a small spot size and alaser having an average output power in this range is sufficient.

In certain embodiments, however, laser 510 can have an output power ashigh as about 100 Watts or more (e.g., up to about 500 Watts). Forexample, in applications where system 500 is used to vaporize tissueover a relatively large area (e.g., several square millimeters orcentimeters), extremely high power lasers may he desirable.

Photonic crystal fiber cars deliver the radiation from laser 510 to thetarget location with relatively high efficiency. For example, the fiberaverage output power can be about 50% or more of the fiber input energy(e.g., about 60% or more, about 70% or more, about 80% or more).Accordingly the fiber's output power can be about 3 Watts or more (e.g.,about 8 Watts or more, about 10 Watts or more, about 15 Watts or more).In certain embodiments, however, the average output power from the fibercan be less than 50% of the laser power, and still be sufficiently highto perform the intended procedure. For example, in some embodiments, thefiber average output power can be from about 20% to about 50% of thelaser average output power.

The length of photonic crystal fiber 520 can vary as desired. In someembodiments, the fiber is about 1.2 meters long or more (e.g., about 1.5meters or more, about 3 meters or more, about 3 meters or more, about 5meters or more). The length is typically dependent on the specific beapplication tor which the laser system is used. In applications wherelaser 510 can be positioned close to the patient, and or where the rangeof motion of the handpiece desired for the application is relativelysmall, the length of the fiber can be relatively short (e.g., about 1.5meters or less, about 1.2 meters or less, about 1 meter or less). Incertain applications, the length of fiber 520 can be very short (e.g.,about 50 centimeters or less, about 20 centimeters or less, about 10centimeters or less). For example, very short lengths of photoniccrystal fiber may be useful in procedures where the system can deliverradiation from the laser to the fiber by some other means (e.g., adifferent waveguide or an articulated arm). Very short fiber lengthsmaybe useful for nose and ear procedures, for example.

However, in applications where it is inconvenient for the laser to beplaced in close proximity to the patient and/or where a large range ofmotion of the handpiece is desired, the length of the fiber is longer(e.g., about 2 meters or more, about 5 meters or more, about 8 meters ormore). For example, in surgical applications, where a large team ofmedical practitioners is needed in close proximity to the patient, itmay be desirable to place the laser away from, the operating table(e.g., in the corner of the operating room, or in a different roomentirely). In such, situations, a longer fiber may be desirable.

In general photonic crystal fiber 520 is flexible, has a bend plane, andcan be bent in relatively small radii of curvature over relatively largeangles without significantly impacting its performance (e.g., withoutcausing the fiber to fail, or without reducing the fiber transmission toa level where the system cannot be used for its intended use while thefiber is bent). In some embodiments, an operator can bend photoniccrystal fiber 520 to have a relatively small radius of curvature, suchas about 15 cm or less (e.g., about 10 cm or less, about 8 cm or less,about 5 cm, or less, about 3 cm or less) while still deliveringsufficient power to the target location for the system to perform itsfunction.

In general, the angle through which the fiber is bent can vary, andusually depends on the procedure being performed. For example, in someembodiments, the fiber can be bent through about 90° or more (e.g.,about 120° or more, about 150° or more),

Losses of transmitted power due to the operator bending photonic crystalfiber 520 may be relatively small. In general, losses due to bendsshould not significantly damage the fiber, e.g., causing it to fail, orreduce the fiber output power to a level where the system can no longerperform the function, for which it is designed. Embodiments of photoniccrystal fiber 520 (e.g., about 1 meter or more in length) can be bentthrough 90° with a bend radius of about 5 centimeters or less, and stilltransmit about 30% or more (e.g., about 50% or more, about 70% or more)of radiation coupled into the fiber at the guided wavelength. Thesefibers can provide such transmission characteristics and provide averageoutput power of about 3 Watts or more (e.g., about 5 Watts or more,about 8 Watts or more, about 10 Watts or more).

The qualify of the beam of the laser radiation emitted from-the outputend of fiber 520 cache relatively good. For example, the beam can have alow M² value, such as about 4 or less (e.g., about 3 or less, about 2.5or less, about 2 or less), M² is a parameter commonly used lo describelaser beam quality, where an M² value of about 1 corresponds to a TEM₀₀beam emitted from a laser, which has a perfect Gaussian profile. The M²value is related to the minimum spot size that can be formed from thebeam according to the formula:

d _(s)=1.27fλM ² /d _(b)

where d_(s) is the minimum spot diameter, d_(b) is the beam diameterprior to being focused to the spot by a lens having focal length f.Accordingly, the minimum, possible spot size a beam can he focused isproportional to the M² value for the beam. Practically, beams havingsmaller values of M² can provide higher radiation power densities to thetarget area, with less damage to surrounding tissue due to the decreasedspot size.

The spot size of radiation delivered by photonic crystal fiber 520 tothe target tissue can be relatively small. For example, in certainembodiments, the spot can have a diameter of about 500 microns or less(e.g., about 300 microns or less, about 200 microns or less, such asabout 100 microns) at a desired working distance from the fiber's outputend (e.g., from about 0.1 mm. to about 3 mm), As discussed previously, asmall spot size is desirable where system 500 is being used to excise orincise tissue or in other applications where substantial precision inthe delivery of the radiation is desired. Alternatively, in applicationswhere tissue is to be ablated or vaporized, and/or a lesser level ofprecision is sufficient, the spot size can be relatively large (e.g.,having a diameter of about 2 millimeters or more, about 3 millimeters ormore, about 4 millimeters or more).

While laser 510 is a CO₂ laser, photonic crystal fibers can be used inmedical laser systems that use other types or lasers, operating atwavelengths different from 10.6 microns. In general, medical lasersystems can provide radiation at ultraviolet (UV), visible, or infrared(IR) Wavelengths. Lasers delivering IR radiation,, for example, emitradiation having a wavelength between about 0.7 microns and about 20microns (e.g., between about 2 to about 5 microns or between about 8 toabout 12 microns). Waveguides having hollow cores, such as photoniccrystal fiber 520, are well-suited for use with laser systems havingwavelengths of about 2 microns or more, since gases that commonly occupythe core have relatively low absorptions at these wavelengths comparedto many dielectric materials (e.g. silica-based glasses and variouspolymers). In addition, to CO₂ lasers, other examples of lasers whichcan emit IR radiation, include Nd:YAG lasers (e.g., at 1.064 microns),Er:YAG lasers (e.g., at 2.94 microns), Er, Cr: YSGG (Erbium, Chromiumdoped Yttrium Scandium Gallium Garnet) lasers (e.g., at 2.796 microns),Ho:YAG lasers (e.g., at 2.1 microns), free electron lasers (e.g., in the6 to 7 micron range), and quantum, cascade lasers (e.g., in the 3 to 5micron range).

In general, the type of laser used in a medical laser system depends onthe purpose for which the system is designed. The type of laser can beselected depending on whether the system is to be used in surgicalprocedures, in diagnosis, or in physiologic studies. For example, anargon laser, which delivers in the blue and green regions of the visiblelight spectrum, with two energy peaks, at 488 nm and 514 nm, can be usedfor photocoagulation. A dye laser, which is a laser with organic dyedissolved in a solvent as the active medium whose beam is in the visiblelight spectrum, can be used in photodynamic therapy. Excimer lasersprovide radiation in the ultraviolet spectrum, penetrates tissues only asmall distance, can he used to break chemical bonds of molecules intissue instead of generating heat to destroy tissue. Such lasers can beused in ophthalmological procedures and laser angioplasty. Ho:YAG laserscan provide radiation in the near infrared spectrum and can be used forphotocoagulation and photoablation. Krypton lasers provide radiation inthe yellow-red visible light spectrum, and can be used forphotocoagulation. Radiation from KTP lasers can be frequency-doubled toprovide radiation in the green visible light spectrum and can be usedfor photoablation and photocoagulation. Nd:YAG lasers can be forphotocoagulation and photoablation. Pulsed dye lasers can be used toprovide in the yellow visible light spectrum (e.g., with a wavelength of577 nm or 585 nm), with alternating on and off phases of a fewmicroseconds each, and can be used to decolorise pigmented lesions.

In general, laser systems can use continuous wave or pulsed lasers.Furthermore, while CO₂ lasers are typically used at average outputpowers of about 5 Watts to about 100 Watts, photonic crystal fibers cangenerally be used with a variety of laser powers. For example, averagelaser power can be in the milliWatt range in certain systems, up to asmuch as several hundred Watts (e.g., about 200 Watts or more) inextremely high power systems.

In general, for high power systems, the average power density guided byfiber 520 can be extremely high. For example, power density in thefiber, or exiting the fiber's core) can be about 10³ W/cm² or more(e.g., about 10⁴ W/cm² or more, about 10⁵ W/cm² or more, 10⁶ W/cm² ormore).

In certain embodiments, handpieces can include actuators that allow theoperator to bend the fiber remotely, e.g., during operation of thesystem. For example, referring to FIG. 6A, in some embodiments, laserradiation 512 can be delivered to target tissue 699 within a patient 601using an endoscope 610. Endoscope 610 includes a gripping portion 611and a flexible conduit 615 connected to each other by an endoscope body616. An imaging cable 622 housing a bundle of optical fibers is threadedthrough a channel in gripping portion 611 and flexible conduit 615.Imaging cable 622 provide illumination to target tissue 699 via flexibleconduit 615. The imaging cable also guides light reflected from thetarget tissue to a controller 620, where it is imaged and displayedproviding visual information to the operator. Alternatively, oradditionally, the endoscope can include an eyepiece lens that allows theoperator to view the target area directly through the imaging cable.

Endoscope 610 also includes an actuator 640 that allow the operator tobend or straighten flexible conduit 615. In some embodiments, actuator640 allows flexible conduit 615 to bend in one plane only, e.g., in thebend plane of fiber 520, Alternatively, in certain embodiments, theactuator allow the flexible conduit to bend in more than one plane.

Endoscope 610 further includes an auxiliary conduit 630 (e.g., adetachable conduit) that includes a channel through, which fiber 520 isthreaded. The channel connects to a second channel in flexible conduit615, allowing fiber 520 to be threaded through the auxiliary conduitinto flexible conduit 615. Fiber 520 is attached to auxiliary conduit ina matter than maintains the orientation of the fiber with respect thechannel through flexible conduit 615, thereby minimizing twisting of thephotonic crystal fiber about its waveguide axis within, the flexibleconduit. In embodiments where photonic crystal fiber 520 has aconfinement region, that includes a seam, the fiber can be attached tothe auxiliary conduit so that the seam is not coincident with a bendplane of the flexible conduit.

In general, photonic crystal fibers can be used in conjunction withcommercially available endoscopes, such as endoscopes available fromPENTAX Medical Company (Montvale, N.J.) and Olympus Surgical &Industrial America, Inc. (Orangeburg, N.Y.).

Auxiliary conduit 630 can be configured to allow the user to extendand/or retract the output end of the photonic crystal fiber withinflexible conduit 615. For example, referring to FIG. 6B, in someembodiments, auxiliary conduit 630 of endoscope 610 can include twoportions 631 and 632 that are moveable with respect to each otherPortion 632 is attached to endoscope body 616, while portion 631telescopes with respect to portion 632. Portion 632 includes a connector636 that connects to a fiber connector 638 attached to fiber 520. Themating mechanism of connector 636 and fiber connector 638 can allow forquick and simple removal and attachment of the photonic crystal fiber tothe endoscope. When attached, connector 636 and fiber connector 638substantially prevent fiber 520 from twisting, maintaining itsorientation about the fiber axis within flexible conduit 615. Theconnectors can maintain the orientation of the fiber in the conduit witha seam in the fiber oriented away from a bend plane of the conduit, forexample. Furthermore, when portion 631 extends or retracts with respectto portion 632, it extends or retracts the output end 645 of fiber 520with respect to the distal end 618 of flexible conduit 615. Auxiliaryconduit 630 also includes a locking mechanism 634 (e.g., a latch orclamp) that allows the user to lock the portion 631 with respect toportion 632. The locking mechanism prevents unwanted movement of fiber520 within flexible conduit 615 while radiation is being delivered tothe patient. The channel in body 616 through which fiber 520 is threadedincludes a kink 650. Connector 638 can be configured so that a seam inthe fiber has a particular orientation with respect to kink 650. Forexample, the seam can be positioned so that it is not on the outside ofthe bend that the fiber experiences at kink 650. In some embodiments,the connectors can orient the seam on the inside of the bend at kink650.

In general, laser systems that utilise photonic crystal fibers can beused in a number of different medical procedures. For example, lasersystems can be used in aesthetic medical procedures, surgical medicalprocedures, ophthalmic procedures, veterinary procedures, and/or dentalprocedures.

Aesthetic procedures include treatment for; hair removal; pulsed lightskin treatments for reducing fine wrinkle lines, sun damage, age spots,freckles, some birthmarks, rosacea, irregular pigmentation, brokencapillaries, benign brown pigment and pigmentation; skin resurfacing;leg veins; vascular lesions; pigmented lesions; acne; psoriasis &vitiligo; and/or cosmetic repigmentation.

Surgical procedures include procedures for gynecology, laparoscopy,condylomas and lesions of the external genitalia, and/or leukoplakia.Surgical applications can also include ear/hose/throat (ENT) procedures,such as laser assisted uvula palatoplasty (LAUP) (i.e., to stopsnoring); procedures to remove nasal obstruction; stapedotomy;tracheobronchial endoscopy; tonsil ablation; and/or removal of benignlaryngeal lesions. Surgical applications can also include breast biopsy,cytoreduction for metastatic disease, treatment of decubitus or stallsulcers, hemorrhoidectomy, laparoscopic surgery, mastectomy, and/orreduction mammoplasty. Surgical procedures can also include proceduresin the field of podiatry, such as treatment of neuromas, periungual,subungual and plantar warts, porokeratoma ablation, and/or radical nailexcision. Other fields of surgery in which lasers may be used includeorthopedics, urology, gastroenterology, and thoracic & pulmonarysurgery.

Ophthalmic uses include treatment of glaucoma, age-related maculardegeneration (AMD), proliferative diabetic retinopathy, retinopathy ofprematurity, retinal tear and detachment, retinal vein occlusion, and/orrefractive surgery treatment to reduce or eliminate refractive errors.

Veterinary uses include both small animal and large animal procedures,

Examples of dental applications include hard tissue, soft tissue, andendodontic procedures. Hard tissue dental procedures include cariesremoval & cavity preparation and laser etching. Soft tissue dentalprocedures include incision, excision & vaporization, treatment of gummysmile, coagulation (hemostasis), exposure of unerupted teeth, aphthousulcers, giogivopiasty, gingivectomy, gingival troughing for crownimpressions, implant exposure, frenectomy, flap surgery, fibromaremoval, operculectomy, incision & drainage of abscesses, oralpapilectomy, reduction of gingival hypertrophy, pre-prosthetic surgery,pericoronitis, peri implantitis, oral, lesions, and sulculardebridement. Endodontic procedures include pulpotomy, root canaldebridement, and cleaning. Dental procedures also include toothwhitening.

Generally, the type of laser, wavelength, fiber length, fiber outerdiameter, and fiber inner diameter, among other system parameters, areselected according to the application. For example, embodiments in whichlaser 510 is a CO₂ laser, laser systems 500 or 600 can be used forsurgical procedures requiring the ablation, vaporization, excision,incision, and coagulation of soft tissue. CO₂ laser systems can be usedfor surgical applications in a variety of medical specialties includingaesthetic specialties (e.g., dermatology and/or plastic surgery),podiatry, otolaryngology (e.g., ENT), gynecology (includinglaparoscopy). neurosurgery orthopedics (e.g., soft tissue orthopedics),arthroscopy (e.g., knee arthroscopy), general and thoracic surgery(including open surgery and endoscopic surgery), dental and oralsurgery, ophthalmology, genitourinary surgery, and veterinary surgery.

In some embodiments, CO₂ laser systems can be used in the ablation,vaporization, excision, incision, and/or coagulation of tissue (e.g.,soft tissue) in dermatology and/or plastic surgery in the performance oflaser skin resurfacing, laser derm-abrasion, and/or laser humdebridement. Laser skin resurfacing (e.g., by ablation and/orvaporization) can be performed, for example, in the treatment ofwrinkles, rhytids, and/or furrows (including fine lines and textureirregularities). Laser skin resurfacing can be performed for thereduction, removal, and/or treatment of: keratoses (including actinickeratosis), seborrhoecae vulgares, seborrheic wart, and/or verrucaseborrheica; vermillionectomy of the lip; cutaneous horns; solar/actinicelastosis; cheilitis (including actinic cheilitis); lentigines(including lentigo maligna or Hutchinson's malignant freckle); unevenpigmentation/dyschromia; acne scars; surgical scars; keloids (includingacne keloidalis nuchae); hemangiomas (including Buccal, port wine and/orpyogenic granulomas/granuloma pyogenicum/granuloma telagiectaticum);tattoos; telangiectasia; removal of skin tumors (including periungualand/or subungual fibromas); superficial pigmented lesions;adenosebaceous hypertrophy and/or sebaceous hyperplasia; rhinophymareduction; cutaneous papilloma; mills; debridement of eczematous and/orinfected skin; basal and squamous cel carcinoma (includingkeratoacanthomas, Bowen's disease, and/or Bowenoid Papulosis lesions);nevi (including spider, epidermal, and/or protruding); neurofibromas;laser de-epithelialization; tricoepitheliomas; xanthelasma palpebrarum;and/or syringoma. CO₂ laser systems can be used for laser ablation,vaporization and/or excision for complete and/or partial nailmatrixectomy, for vaporization and/or coagulation of skin lesions (e.g.,benign aid/or malignant, vascular and/or avascular), and/or for Moh'ssurgery; for lipectomy. Further examples include using laser system 1300for laser incision and/or excision of soft tissue for the performance ofupper and/or lower eyelid blepharoplasty, and/or for the creation ofrecipient sites for hair transplantation.

In certain embodiments, CO₂ laser systems is used in the laser ablation,vaporization, and/or excision of soft tissue during podiatry proceduresfor the reduction, removal, and/or treatment of: verrucaevulgares/plantar warts (including paronychial, periungual, and subungualwarts); porokeratoma ablation; ingrown nail treatment; neuromas/fibromas(including Morton's neuroma): debridement of ulcers; and/or other softtissue lesions. CO₂ laser systems can also be used for the laserablation, vaporisation, author excision in podiatry for complete and/orpartial matrixectomy.

CO₂ laser systems can be used for laser incision, excision, ablation,and/or vaporization of soft tissue in otolaryngology for treatment of:choanal atresia; leukoplakia (including oral, larynx, uvula, palatal,upper lateral pharyngeal tissue); nasal obstruction; adult and/orjuvenile papillomatosis polyps; polypectomy of nose and/or nasalpassages, lymphangioma removal; removal of vocal cord/fold nodules,polyps and cysts; removal of recurrent papillomas in the oral cavity,nasal cavity, larynx, pharynx and trachea (including the uvula, palatal,upper lateral pharyngeal tissue, tongue and vocal cords); laser/tumorsurgery in the larynx, pharynx, nasal, car and oral structures andtissue; Zenker diverticulum/pharynoesopltageal diverticulum (e.g.,endoscopic laser-assisted esophagodiverticulestomy); stenosis (includingsubglottic stenosis); tonsillectomy (including tonsillar cryptolysis,neoplasms) and tonsil ablation/tonsillotomy; pulmonary bronchial andtracheal lesion removal; benign and malignant nodules, tumors andfibromas (e.g., of the larynx, pharynx, trachea,tracheobronchial/endobronchial); benign and/or malignant lesions and/orfibromas (e.g., of the nose or nasal passages); benign and/or malignanttumors and/or fibromas (e.g., oral); stapedotomy/stapedectomy; acousticneuroma in the ear; superficial lesions of the ear (includingchondrodermatitis nondularis chronica helices/Winkler's disease);telangiectasia/hemangioma of larynx, pharynx, and/or trachea (includinguvula, palatal, and/or upper lateral pharyngeal tissue); cordectomy,cordotomy (e.g., for the treatment of vocal cord paralysis/vocal foldmotion impairment), and/or cordal lesions of larynx, pharynx, and/ortrachea; myringotomy/tympanostomy (e.g., tympanic membranefenestration); uvulopalastoplasty (e.g., LAUP); turbinectomy and/orturbinate reduction/ablation; septal spur ablation/reduction and/orseptoplasty; partial glossectomy; tumor resection on oral, subfacialand/or neck tissues; rhinophyma; verrucae vulgares; and/orgingivoplasty/gingivectomy.

In some embodiments, CO₂ laser systems can be used for the laserincision, excision, ablation, and/or vaporization of soft tissue ingynecology for treatment of: conizaton of the cervix (including cervicalintraepithelial neoplasia, vulvar and/or vaginal intraepithelialneoplasia); condyloma acuminata (including cervical, genital, vulvar,preineal, and/or Bowen's disease, and/or Bowenoid papulosa lesions);leukoplakia (e.g., vulvar dystrophies); incision and drainage ofBartholin's and/or nuhuthlan cysts; herpes vaporization; urethralcaruncle vaporisation; cervical dysplasia; benign and/or malignanttumors; and/or hemangiomas.

CO₂ laser systems can he used for the vaporisation, incision, excision,ablation and/or coagulation of soft tisane in endoscopic and/orlaparoscopic surgery, including gynecology laparoscopy, for treatmentof: endometrial lesions (inclosing ablation of endometriosis);excision/lysis of adhesions; salpingostomy; oophorectomy/ovariectomy;fimbroplasty; metroplasty; tubal microsurgery; uterine myomas and/orfibroids; ovarian fibromas and/or follicle cysts; uterosacral ligamentablation; and/or hysterectomy.

In certain embodiments, CO₂ laser systems are used for the laserincision, excision, ablation, and/or vaporization of soft tissue inneurosurgery for the treatment of cranial conditions, including;posterior fossa tumors; peripheral neurectomy; benign and/or malignanttumors and/or cysts (e.g., gliomos, menigiomas, acoustic neuromas,lipomas, and/or large tumors); arteriovenous malformation; and/orpituitary gland tumors. In some embodiments, CO₂ laser systems are usedfor the laser incision, excision, ablation, and/or vaporization of softtissue in neurosurgery for the treatment of spinal cord conditions,including: incision/excision and/or vaporization of benign and/ormalignant tumors and/or cysts; intra- and/or extradural lesions; and/orlaminectomy/laminotomy/micordisectomy.

CO₂ laser systems cars be used for the incision, excision, and/orvaporization of soft tissue in orthopedic surgery in applications thatinclude arthroscopic and/or general surgery. Arthroscopic applicationsinclude; menisectomy; chondromalacia; chondroplasty; ligament release(e.g., lateral ligament release); excision of plica; and/or partialsynovectomy. General surgery applications include: debridement oftraumatic wounds; debridement of decubitus and/or diabetic ulcers;microsurgery; artificial joint revision; and/or polymer (e.g.,polymethylmethacrylate) removal.

CO₂ laser systems can also be used for incision, excision, and/orvaporization of soil tissue in general and/or thoracic surgery,including endoscopic and/or open procedures. Such applications include:debridement of decubitus ulcers, stasis, diabetic and other ulcers;mastectomy; debridement of burns; rectal and/or anal hemorrhoidectomy;breast biopsy; reduction mammoplasty; cytoreduction for metastaticdisease; laparotomy and/or laparoscopic applications; mediastinal and/orthoracis lesions and/or abnormalities; skin tag vaporization; atheroma;cysts (including sebaceous cysts, pilar cysts, and/or mucous cysts ofthe lips); pilonidal cyst removal and/or repair; abscesses; and/or othersoft tissue applications.

In certain embodiments, CO₂ laser systems can be used for the incision,excision, and/or vaporization of soft tissue in dentistry and/or oralsurgery, including for: gingivectomy; gingivoplasty; incisional and/orexcisional biopsy; treatment of ulcerous lesions (including aphthousulcers); incision of infection when used with, antibiotic therapy;frenectomy; excision and/or ablation of benign and/or malignant lesions;homeostasis; operculectomy; crown lengthening; removal of soft tissue,cysts, and/or tumors; oral cavity tumors and/or hemangiomas; abscesses;extraction site hemostasis; salivary gland pathologies; preprostheticgum preparation; leukoplakia; partial glosseotomy; and/or periodontalgum resection.

In some embodiments, CO₂ laser systems can be used for incision,excision, and/or vaporization of soft tissue in genitourinaryprocedures, including for: benign and/or malignant lesions of externalgenitalia; condyloma; phimosis; and/or erythroplasia.

In addition to medical applications, photonic crystal fibers such asthose described herein can be used in other applications as well. Insome embodiments, photonic crystal fibers can be used to guide radiationbetween a source and a detector. FIG. 7 shows a schematic diagram of asystem 700 including a source 710 and a detector 720, which are coupledto one another by a photonic crystal fiber 730. In certain embodiments,system 700 is an optical telecommunication system and photonic crystalfiber 730 serves as an optical transmission line to guide opticalsignals between source 710 and detection system 720. In general, theoptical transmission line may include one or more other segments inaddition to photonic crystal fiber 730. Source 710 may be the originalsource of an optical, signal directed along the transmission line or itmay be an intermediate node that redirects the optical signal to thetransmission line, optically amplifies it, and/or electronically detectsit and optically regenerates it. Furthermore, source 710 may includecomponents for multiplexing or demultiplexing multiple optical signalsat different wavelengths. Similarly, detector 720 may be the finaldestination for the optical signal transmitted along the transmissionline, or it may be an intermediate node that redirects, opticallyamplifies, and/or electrically detects and optically regenerates theoptical signal. In addition, defector 720 may also include componentsfor multiplexing or demultiplexing multiple optical signals at differentwavelengths. The optical signal transmitted along the transmission linemay be a WDM signal that includes multiple signals at correspondingwavelengths. Suitable wavelengths for the system include those within arange of about 1.2 microns to about 1.7 microns, which corresponds tomany long-haul, systems in use today, as well those within a range ofabout 0.7 microns to about 0.9 microns, which corresponds to some metrosystems currently being considered.

Because of their small losses, the photonic crystal fibers describedherein may provide one or more advantages when used as the transmissionfiber in an optical telecommunications system. Because the losses aresmall, the lengths of the transmission line can be made larger asperiodic amplification is less necessary. For example, the losses may besmaller than 1 dB/km, smaller than 0.1 dB/km, or even, smaller than 0.01dB/km. Moreover, because FWM is reduced, WDM channel spacing in thefiber can be made smaller.

In some embodiments, system 700 may be a diagnostic tool For example,photonic crystal fiber 730 can be used as a sample cell in a gas-phasespectrometer, where the hollow core of fiber 730 is filled with a samplegas. Radiation launched into fiber 730 interacts with the gas.Typically, the amount of radiation at different wavelengths depends onrite composition of the gas in the core. Thus, by monitoring theintensity of radiation exiting the fiber at different wavelengths, onecan determine the composition of the gas. In such embodiments, detector720 can be connected to a processor (e.g., a computer), which performsan analysis of a signal generated by detector 720 in response toradiation from the source. An example of a gas phase spectrometerutilizing a hollow fiber is described by C. Charlton et al, in IEEEProc.-Optoelectron., Vol. 150, No. 4, pp. 306-309.

In some embodiments, a photonic crystal fiber, such as those describedabove, can be used to deliver laser radiation to a target. For example,referring to FIG. 8, a laser system 800 includes a laser 810 and aphotonic crystal fiber 820 for guiding electromagnetic (EM) energy fromthe laser to a target 830 (e.g., a sheet of steel or a patient) remotefrom the laser. Radiation is coupled from laser 810 into fiber 820 usinga coupler 840. Laser system 800 also includes a focusing element 850(e.g., a lens or combination of lenses) that focuses radiation 801emerging from photonic crystal fiber 820 onto target 830. The radiationcan, for example, be used to cut, clean, ablate, coagulate, form,liquefy, engrave and/or weld material at target 830. For example, informing applications, laser radiation can be directed to a metal sheetin order to thermal stress a portion of the sheet, which causes thesheet to bend.

Laser 810 can be a continuous wave or pulsed laser. The distance betweenlaser 810 and target 830 can vary depending on the specific application,and can be on the order of several meters or more (e.g., about 10 m ormore, about 20 m or more, about 50 m or more, about 100 m or more).

Laser system 800 can operate at UV, visible, or infrared (IR)wavelengths. In some embodiments, photonic crystal fiber 820 isconfigured to guide IR energy emitted by laser 810, and the energy has awavelength between about 0.7 microns and 20 microns (e.g., between about2 to 5 microns or between about 8 to 12 microns). In some embodiments,laser 1210 is a CO₂ laser and the radiation has a wavelength of about6.5 microns or 10.6 microns. Other examples of lasers which can emit IR.energy include Nd:YAG lasers (e.g., at 1.064 microns) Er:YAG lasers(e.g., at 2.94 microns), Er, Cr: YSGG (Erbium, Chromium doped YttriumScandium Gallium Garnet) lasers (e.g., at 2.796 microns), Ho:YAG lasers(e.g., at 2.1 microns), free electron lasers (e.g., in the 6 to 7 micronrange), and quantum cascade lasers (e.g., in the 3 to 5 micron range).

The power emitted from laser 810 at the guided wavelength can vary.Although the laser power can be relatively low, e.g., mW, in manyapplications the laser system is operated at high powers. For example,the laser output intensity can be about one Watt or more (e.g., aboutfive Watts or more, about 10 Watts or more, about 20 Watts or more). Insome applications, the laser output energy can be about 100 Watts ormore (e.g., about 200 Watts or more, about 300 Watts or more, about 500Watts or more, about 1 kilowatt or more).

For high, power systems, the power density guided by fiber 820 can berelatively high. For example, power density in the fiber can be about10⁵ W/cm² or more, such as about 10⁶ W/cm² or more, about 10⁷ W/cm² ormore, about 10 ⁸ W/cm² or more, about 10⁹ W/cm² or more, about 10¹⁰W/cm² or more.

Fiber 1820 can have relatively low losses at the guided wavelength(e.g., about 10 dB/m or less, about 5 dB/m or less, about 2 dB/m orless, about 1 dB/m or less, about 0.5 dB/m or less, about 0.2 dB/m orless). Due to the low loss, only a relatively small amount of the guidedenergy is absorbed by the fiber, allowing the fiber to guide high powerradiation, without substantial damage due to heating.

Coupler 840 can be any coupler suitable for the wavelength and intensityat which the laser system operates. One type of a coupler is describedby R. Nubling and J Harrington in “Hollow-waveguide rich very systemsfor high-power, industrial CO₂ lasers,” Applied Optics, 34, No. 3, pp.372-380 (1996). Other examples of couplers include one or more focusingelements, such as one or more lenses. Coupling efficiency can be high.For example, coupler 140 can couple about 70% or more of the laseroutput into a guided mode in the fiber (e.g., about 80% or more, about90% or more, about 95% or more, about 98% or more). Coupling efficiencyrefers to the ratio of power guided away by the desired mode to thetotal power incident on the fiber.

Other embodiments are within the scope of the following claims.

1.-25. (canceled)
 26. A waveguide fiber, comprising: a core extendingalong a waveguide axis; a confinement region extending along thewaveguide axis, the confinement region surrounding the core; and acladding extending along the waveguide axis, the cladding surroundingthe confinement region, wherein the waveguide fiber bends preferably ina bend plane relative to other planes, and the cladding includes a firstportion extending along the waveguide axis and a second portionextending along the waveguide axis, the first portion being composed ofa first material having a first stiffness and the second portion beingcomposed of a second material having a second stiffness, the firststiffness being different from the second stiffness.
 27. (canceled) 28.The waveguide fiber of claim 26 wherein the cladding further comprises athird portion being composed of a third material having a thirdstiffness, the third stiffness being different from the first stiffness.29. The waveguide fiber of claim 28 wherein, in cross-section, the coreis positioned between the second portion and the third portion.
 30. Thewaveguide fiber of claim 26 wherein the first portion surrounds thesecond portion. 31.-33. (canceled)
 34. The waveguide fiber of claim 26,wherein the first stiffness is greater than the second stiffness. 35.The waveguide fiber of claim 26, wherein the first stiffness is lowerthan the second stiffness.
 36. The waveguide fiber of claim 26, whereinthe waveguide fiber comprises a photonic crystal fiber.
 37. A system,comprising: a CO₂ laser; and a waveguide fiber, the waveguide fiberhaving an input end positioned relative to the CO₂ laser to receiveradiation from the CO₂ laser and the waveguide fiber is adapted todeliver the radiation to a target, wherein the waveguide fibercomprises: a core extending along a waveguide axis; a confinement regionextending along the waveguide axis, the confinement region surroundingthe core; and a cladding extending along the waveguide axis, thecladding surrounding the confinement region, wherein the cladding has anasymmetric cross-section that extends along a length of the waveguidefiber.
 38. The system of claim 37, wherein the waveguide fiber comprisesa photonic crystal fiber.
 39. A system, comprising: a waveguide fiberhaving an input end and an output end; and a handpiece attached to thewaveguide fiber, wherein the handpiece allows an operator to control theorientation of the output end to direct the radiation to a targetlocation of a patient and the waveguide fiber comprises: a coreextending along a waveguide axis; a confinement region extending alongthe waveguide axis; the confinement region surrounding the core; and acladding extending along the waveguide axis, the cladding surroundingthe confinement region, wherein the cladding has an asymmetriccross-section that extends along a length of the waveguide fiber. 40.The system of claim 39 wherein the handpiece comprises an endoscope. 41.The system of claim 40 wherein the endoscope comprises a flexibleconduit and a portion of the waveguide fiber is threaded through achannel in the flexible conduit.
 42. The system of claim 41 wherein theendoscope comprises an actuator mechanically coupled to the flexibleconduit configured to bend a portion of the flexible conduit in at leastone plane thereby allowing the operator to vary the orientation of theoutput end.
 43. The system of claim 42 wherein the waveguide fiber isattached to the endoscope so that the at least one plane corresponds tothe bend plane of the waveguide fiber.
 44. The system of claim 39,wherein the waveguide fiber comprises a photonic crystal fiber.
 45. Afiber assembly comprising: a jacket adapted to surround a waveguidefiber having a waveguide axis, the jacket exhibiting a preferential bendplane.
 46. The fiber assembly of claim 45, wherein the jacket comprisesan asymmetric cross-sectional profile about the waveguide axis.
 47. Thefiber assembly of claim 46, wherein the cross-sectional profile iselliptical.
 48. The fiber assembly of claim 45, further comprising: thewaveguide fiber disposed within the jacket.
 49. The fiber assembly ofclaim 48, wherein the waveguide fiber comprises a core extending alongthe waveguide axis.
 50. The fiber assembly of claim 49, wherein thewaveguide fiber comprises a confinement region extending along thewaveguide axis, the confinement region surrounding the core.
 51. Thefiber assembly of claim 50, wherein the confinement region comprises aseam.
 52. The fiber assembly of claim 51, the waveguide fiber furthercomprises a cladding extending along the waveguide axis, the claddingsurrounding the confinement region.
 53. The fiber assembly of claim 52,wherein the cladding is adhesively bonded to the jacket.
 54. The fiberassembly of claim 53, wherein the jacket and cladding are consolidated.55. The fiber assembly of claim 48, wherein the waveguide fibercomprises a photonic crystal fiber.
 56. A system comprising: a CO₂laser; and the fiber assembly comprising: a waveguide fiber having awaveguide axis, and a jacket surrounding the waveguide fiber, the jacketexhibiting a preferential bend plane, wherein the fiber assembly has aninput end positioned relative to the CO₂ laser to receive radiation fromthe CO₂ laser and the laser assembly is adapted to deliver the radiationto a target.
 57. The system of claim 50, wherein the jacket comprises anasymmetric cross-sectional profile about the waveguide axis.
 58. Thesystem of claim 56, wherein the waveguide fiber comprises a photoniccrystal fiber.